Uncle Al's 2017-2020 Racing Rules in Pics
Definitions are quoted, then illustrated and explained as seems necessary.
for instant access without scrolling, click on the selected definitions list below
Leeward and Windward
Tack, Starboard or Port
Clear Astern and Clear Ahead; Overlap
Keep Clear
Proper Course
There are more definitions but these do not lend themselves easily to illustration.
For the complete list and text of Definitions in the RRS,  click here.

Leeward and Windward A boat's leeward side is the side that is or, when she is head to wind, was away from the wind. However, when sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies. The other side is her windward side. When two boats on the same tack overlap, the one on the leeward side of the other is the leeward boat. The other is the windward boat. 

In essence, the leeward side of the boat is the side on which the mainsail is (or was, if you're head to wind). The other side is - obviously? - the windward side.

Since Heider Funck in W6 is overlapped with W3854 (Uncle Al) and off Al's leeward side, W6 is the leeward boat and Uncle Al is the windward boat - oddly enough.
Tack, Starboard or Port A boat is on the tack, starboard or port, corresponding to her windward side.

When you face the bow of the boat, the side to your right is the starboard side, and the left side is the port side. Here the left (port) side of John Weakley's Opti is her windward side, and she is thus on port tack.
Clear Astern and Clear Ahead; Overlap  One boat is clear astern of another when her hull and equipment in normal position are behind a line abeam from the aftermost point of the other boat's hull and equipment in normal position. The other boat is clear ahead. They overlap when neither is clear astern. However, they also overlap when a boat between them overlaps both. These terms always apply to boats on the same tack. They apply to boats on opposite tacks only when rule 18 applies between them or when both boats are sailing more than ninety degrees from the true wind.

W7663 is clear ahead of Laser 61007 and W286 since the latter are behind an imaginary line drawn at a 90º angle to the centreline of W7663 (i.e. abeam) through the aftermost tip of W7663's rudder blade (i.e. the aftermost point of her hull and equipment in normal position). Even if a brightly visible spinnaker sheet, for example, was dragging several metres behind W7663, the other two boats would still not be overlapped with 7663 since that would be equipment not in its normal position.

Y-Flyer 1760 (r) is overlapped with Peter Rahn (black hull) since not all of his hull and equipment are aft of the (imaginary) line drawn abeam from the aftermost point of Peter's hull and equipment (in normal position).

The red vertical line through Hans Gottschling's spinnaker in the above photo illustrates that the bow is not necessarily the forewardmost part of a boat's hull and equipment. For example, on a spinnaker run, a boat whose bow is a few centimetres short of gaining an overlap, may well have that crucial overlap due to her spinnaker.

The sharp turn Uncle Al is initiating on the starting line above, will soon give Doug Netherton (left) a leeward overlap as the red line swings around dramatically with the aft part of Al's boat.

Above, Uncle Al (9355) has an overlap on Søren Jensen (10212) because Sten Madsen (9067) is between them and overlaps both 10212 and 9355. Just in time to get buoy room it was, too!!
In the photo on the left above, W6732 and W286 are overlapped even though they are on opposite tacks because both are "sailing more than ninety degrees from the true wind".
Once W9235 and W7700 (above right) reach the zone around the red mark I have "drawn in" (it was down there somewhere - I know! I nearly hit it!!) the term overlap can be applied between them because Rule 18 applies. So, if Roger Shepherd (7700) can just turn on the afterburners, he can get an overlap in time to claim "buoy room/water" from Brian Jeffs (9235).
Racing A boat is racing from her preparatory signal until she finishes and clears the finishing line and marks or retires, or until  the  race  committee  signals  a  general  recall, postponement, or abandonment.
Al's note: This definition is important since the the preamble to Part 2 of  the RRS (right-of-way rules and such like) specifies that "a boat not racing shall not be penalized for breaking one of these rules, except rule 24.1 ". For the common good, we are of course expected to obey all these Rules whenever we are in the race area - even when we can't be penalized for breaking them because we're not actually racing.

Above, W864 (Nick, blue hull) has just tacked across the finish line and has finished. But he has not yet "cleared the finish line" and is therefore still racing. If Nick had tacked a second or two later, he might have fouled Gale Shoemaker (red hull).  In that case, Nick would have had to do his penalty turns and finish one more time - this one to count!! Likewise, a 360 and a re-finish will be required, if Nick should hit the finish mark - even after getting his “beep”.
Al's note: There is no longer a definition of "tacking" in our "new" Rules. The old Rules more or less defined tacking as the period of time from the moment a boat turns beyond head to wind until she is on a close-hauled course. That definition must now be inferred from Rule 13 as follows:

13  While Tacking
After a boat passes head to wind, she shall keep clear of other boats until she is on a close-hauled course....

For illustrated coverage of Rule 13, click here.
Room The space a boat needs in the existing conditions, including space to comply with her obligations under the rules of Part 2 and rule 31, while manoeuvring promptly in a seamanlike way.  
Mark-Room  Room for a boat to leave a mark on the required side. Also,
(a) room to sail to the mark when her proper course is to sail close to it, and
(b) room to round the mark as necessary to sail the course.
However, mark-room for a boat does not include room to tack unless she is overlapped inside and to windward of the boat required to give mark-room and she would be fetching the mark after her tack.

In the photo above, W825 (Ed Tait) is the leeward boat and has right-of-way. Since CL523 (Colin Junkin) has an inside overlap however, 825 is required by Rule 18 to give 523 mark-room. This means that 523 is expected by the Rules and the above definitions of room and mark-room, to sail to the mark in a seamanlike manner, i.e. keep his boat under reasonable control for the conditions, get his board down in time for the upcoming beat, get his sails in promptly and round as close to the mark as prudence and good seamanship would indicate in the prevailing conditions. In the conditions above, with the mark and the boats bouncing around, 523 is entitled to more room than he would be in 5 knots of breeze, flat water and no current. Once at the mark, the new mark-room definition entitles 523 to sail proper course until he is no longer "at the mark", at which point 523's mark-room protection ends and he must again keep clear of any right-of-way boat(s).
Keep Clear A boat keeps clear of a right-of-way boat
(a) if the right-of-way boat can sail her course with no need to take avoiding action and,
(b) when the boats are overlapped, if the right-of-way boat can also change course in both directions without immediately making contact.

Above, Len Macdougall in Wayfarer 6732 (on port tack) has kept clear of Nick Seraphinoff in W864 who had no need to take avoiding action. W2960, Don Thwing, on the other hand, has reached the danger zone: Within the next second or so, Nick in 864 will have to begin avoiding action or face breaking Rule 14 in a possible collision with 2960. Unless 2960 tacks immediately (or far more riskily, bears away radically now), he will not be keeping clear of the starboard boat, 864, and will have fouled the latter.

In the photo above, Marc Bennett (W6) is keeping clear of Tanya Wharton in the white-hulled Wayfarer to leeward of him, but Tanya has sailed so close to W3854 that Uncle Al can no longer "change course in both directions without immediately making contact with the windward boat". Ergo, Tanya is not keeping clear of 3854 and has fouled Uncle Al under Rule 11.
Proper Course A course a boat would sail to finish as soon as possible in the absence of the other boats referred to in the rule using the term. A boat has no proper course before her starting signal.

There is a widespread belief that proper course is the same thing as sailing the rhumbline (i.e. straight towards the next mark) on anything except an upwind leg of the course. This, in fact, is rarely the case. In the photo above for example, Rule 17 requires Colin in CL523 not to sail above his proper course because he became overlapped from clear astern within two lengths of Ed (W825). But that does not necessarily mean Colin must sail straight at the next mark. He is simply required to sail the course that he feels would get him around the course in the fastest possible time "in the absence of the other boats referred to in the Rule using the term", i.e. W825 would be the "other boat" (the windward boat, in this case) referred to in Rule 17.

Here, Colin (CL523) can defend sailing high of the mark if he can convince the Protest Committee that because the spinnaker reach was quite close, he has been sailing high of the mark for some time since he wanted to make sure he would lay it. In other words, he would have sailed above the direct course to the next mark, even if Ed in W825 had not been there.

Before the start, we have no particular place to go, and there is, logically enough, no proper course before the start signal is made. Therefore, Rule 17 which requires sailing no higher than proper course in some circumstances, does not "kick in" until after the start signal has been made.

Upwind, proper course is basically any close-hauled course that a boat may choose to sail. Above, Chris Lansdown in W4678 (the leeward boat) can pinch as high as he likes and Gary McIlroy in CL2679 (windward) will have to keep clear.

Worrying about sailing proper course upwind usually only happens just after the start: Let us assume for example, that these two have just started and that 4678 established a leeward overlap from clear astern while 2679 was "sitting" on the line with her sails luffing and waiting for the "gun". Once the gun went, 4678 became subject to Rule 17 which limits 4678 to sailing no higher than his proper course. Thus, W4678  had to bear away to close-hauled as soon as the start signal was made but is entitled to sail her proper course. If that is higher than 2679 is able to point, tough beans!! (Al's note: this is not the official ISAF terminology)
Obstruction An object that a boat could not pass without changing course substantially, if she were sailing towards it and one of her hull lengths from it. An object that can be safely passed on only one side and an area so designated by the sailing instructions are also obstructions. However, a boat racing is not an obstruction to other boats unless they are required to keep clear of her, give her room or mark-room or, if rule 23 applies, avoid her. A vessel under way, including a boat racing, is never a continuing obstruction.

Apart from the obvious but rarely met obstructions such as piers sticking out in front of us or a ferry coming across, the obstructions that we face most regularly are right-of-way boats of which we must keep clear, boats to whom we must give mark-room, or capsized boats. The photo above illustrates a very common situation: 1305 is on starboard. Since both port boats will very soon have to take avoiding action for 1305, she ranks as an obstruction to both. The leeward port boat (1131) could have called for "room to tack" for the obstruction (see Rule 20) or chosen to pass astern of the obstruction (1305). If 1131 chooses the latter course, she must - under Rule 19 - give room to 171 if the latter also wants to pass astern of 1305. It's very much as if 1305 were a moving mark.
Fetching  A boat is fetching a mark when she is in a position to pass to windward of it and leave it on the required side without changing tack.
Fetching the mark is the same as “laying” it, i.e. being in a position to round the mark without having to put in any more tacks.

Fetching??!! Fetching :)
You want fetching, I’ll give you fetching.
The young ladies of the social committee (left) at the 2012 inaugural Bayview YC One-Design Regatta are very fetching indeed!”

Or of course, Fido and Rover are familiar with the word “Fetch”.

Actually, “fetch” is what the Brits (and others?) say instead of “lay” - as in “Can we lay it yet?” Even being Canadian, I do find “fetch” useful though, when I need to use it as a noun, e.g. “The beat had turned into a fetch.” (= one-tack beat) where the noun “lay” might sound odd to some.

Greenwood Lake Rebel sailors, Bob and Jean Zimmer, look like they are fetching/laying the mark at Rock Hall 2012.
Finish  A boat finishes when any part of her hull, or crew or equipment in normal position, crosses the finishing line from the course side. However, she has not finished if after crossing the finishing line she
(a) takes a penalty under rule 44.2,
(b) corrects an error under rule 28.2 made at the line, or
(c) continues to sail the course.

Stephan Nandrup-Bus (W4898) finished just a split second ago when the first part of his spinnaker broke the plane of the finish line. If his spinnaker had been allowed to flap loose to fly further forward, or if his crew - God forbid - had been hanging off the bow with an arm stretched forward, that would not have been gear in normal position and the finish would have been taken at the moment the first part of his boat that was in normal position crossed the line - likely his little mini-bowsprit (spinnaker sheet catcher).

If this were a "buoys to starboard" course, then this RC boat would be anchored on the "wrong" side for a "buoys to starboard" finish. This happens frequently for a variety of reasons, and is not, in itself, considered grounds for redress. The finish definition makes it clear that in such an instance, the side the mark was to be left on no longer matters, since a boat finishes when it "crosses the finishing line in the direction of the course from the last mark". Thus CL1050 (Rob Wierdsma with son, Ben) is finishing correctly in the photo above, regardless of whether it was a buoys to port or starboard course.
Rules in Pictures index

Part 2A
Part 2B
Part 2C
Part 2D + penalty turns

Photo-based Rules Quiz - 1
Photo-based Rules Quiz - 2