What's the WOD ?

WOD is "Workout of the Day." The great folks at CrossFit post a workout for each day. The common splits are a) as posted, which is 3 days on/1 off, and b) 5 days on, two off.
If you need explanation on doing the WODs, check here or in the discussion board. Likely your question has been asked before.

Some insight and thoughts on sets and reps:

The WOD descriptions are very literal; don't read into them. If it says "squats" it means bodyweight (aka "air squats") - no added weight, unless it says back squats or front squats.
A "rep" or repetition is one iteration of a movement. One bench press, one squat. A "set" is a group of reps: 10 reps =10 bench presses, 10 squats. 3 sets is do a group of repetitions, rest, repeat, rest, repeat. So, 3 sets of 10 (reps) is 10/rest/10/rest/10. The rest interval is up to your recovery time, and the goal of the WOD. Obviously, if it's a timed WOD, you want to rest less.
Also, rest and reps are frequently inverse. Sometimes a WOD says deadlift 3-2-2-1-1-1. This means a set of 3 reps, a set of 2 reps, another set of 2, a "set of one" aka a "single." This few reps indicates maximal load, and indicates longer rest times.
Back to literal: if the WOD says 21-15-9 reps of bench and pullups in "rounds" (or any two or three exercises as given) you do 21 reps of exercise 1, followed by 21 reps of exercise 2, and 21 reps of exercise 3 if there is a third one. Now do 15 of the first, 15 of the second...9 of the first, 9 of the second.
Most likely you will be breaking the 21's and 15's (and maybe the 9's) into subsets, aka "breakdowns." This is based on your strength and conditioning. Remember if you need to adjust the weight downward, do so, since these are timed WODs.

Here's some insight from Coach on the intent of CrossFit:

"CrossFit is in large part derived from several simple observations garnered through hanging out with athletes for thirty years and willingness, if not eagerness, to experiment coupled with a total disregard for conventional wisdom. Let me share some of the more formative of these observations:
1. Gymnasts learn new sports faster than other athletes.
2. Olympic lifters can apply more useful power to more activities than other athletes.
3. Powerlifters are stronger than other athletes.
4. Sprinters can match the cardiovascular performance of endurance athletes - even at extended efforts.
5. Endurance athletes are woefully lacking in total physical capacity.
6. With high carb diets you either get fat or weak.
7. Bodybuilders can't punch, jump, run, or throw like athletes can.
8. Segmenting training efforts delivers a segmented capacity.
9. Optimizing physical capacity requires training at unsustainable intensities.
10. The world's most successful athletes and coaches rely on exercise science the way deer hunters rely on the accordion."

Source : http://www.crossfit.com/cf-info/faq.html