Hare Scramble
Closed-course off-road race in which riders go fast for 2-3 hours on
a course laid out primarily in the woods.  Course length normally
varies between 3 and 20 miles and includes all sorts of fun stuff like
mud, steep hills, rocks, trees, creeks, rivers, and lots of crashing
(well, that's mostly me). The object is to complete the most laps
within the time period, and as long as you reach the main scoring
area prior to the time cut-off, then you keep doing laps until one of
the following occurs:

1. Time runs out
2. You break something important on your bike and quit
3. You break something important on your body and quit
4. Combination of 2 and 3 above
5. You run out of energy and say "I will be leaving now"

Enduro
Off-road race in which the goal is to maintain an average speed
(defined by the promoters) over the entire course.  Course length is
often a 30-40 mile loop run twice, but can be longer.  Riders are
checked at various points throughout the course (riders almost
never know where the checks are located) and points are added to
the overall score if a rider is early or late.  Penalties for being early
are far more severe than for being late.  The object is to receive the
lowest score, kind of like golf except it's not boring.

Motocross
An off-road sport with origins in Southern California, also known as
Land of People Enjoying Spinal Injuries.  The courses involve many
sharp turns and large jumps, and most are set up with laps that the
fast guys can do in 2-3 minutes.  Spectators can usually view most,
if not all, of the crashing, I mean racing, from the same vantage
point.  Indoor motocross is called Supercross or ArenaCross.

Freestyle Motocross (FMX)
This motocross derivative has been popularized by Travis Pastrana
and ESPN's X-Games.  The object of this competition is to jump
huge and while in mid-air, remove your body as far as possible from
the motorcycle and get back on the bike before landing.  At a FMX
competition, Travis Pastrana once separated his spine from his
pelvis.  That is correct, HE SEPARATED HIS SPINE FROM HIS
PELVIS.  During a 6-month recovery period, Travis was filmed
bragging that he had his wheelchair up to 25 mph (or therabout).  
Travis finally lost an FMX competition in 2003 after he knocked
himself silly attempting a backflip/360 combo. That is correct, HE
FLIPPED HIS BIKE UPSIDE DOWN WHILE ROTATING THE BIKE
360 DEGREES IN MID-AIR, WITH HIS BODY STILL ATTACHED TO
THE MOTORCYCLE.

Classes
Pretty simple, really.  You've got your fast guys, your slow guys,
and the guys in between.  The A class is  fast guys, the C class is
the slow guys, and the B class is intermediate level.  Also, there's a
AA class for the insanely fast pro riders.  The classes are then
divided between engine sizes, usually small bikes (up to 200cc),
medium sized bikes (201-250cc) and big bikes (over 250cc).  Then
there's a special class for 4-stroke engines.  Then there's age
groups, mostly for more "mature" guys, like Vet, Senior, Super
Senior, Super-Duper Extra Special Senior, and the very popular Hip
Replacement Class.  Then there's classes for mini-bikes, women,
and beginners, and for people who just want to sit by the trail and
drink beer.  So basically, each event has exactly 247 classes and
the only guys who go home without trophies are white males in their
late-20's with German heritage and dark hair riding orange
European dirt bikes with 300cc engines and Michelin front tires.
Off-road Racing Terms Defined