[EricH]

I'm with Sean here. There are many different types of swerve/crab drive.

Methods of driving and turning the wheels (non-split or split):
-Coaxial--the same axle used for turning the wheel assembly also has an axle to power the wheel (118, 148)
-Independent--each wheel has its own motor, no matter what is used to turn the assembly (16)

Then you get the other tricks--how to turn the wheel assemblies.
-Independent--each assembly has its own motor for turning. I believe 16 did this in 2006 with their three-wheel swerve.
-All together--often used with coaxial swerve. The true swerve drive; you can't turn your robot (well, you could, but it would need some interesting drive software) so you need a turret if you are going to need to score at other angles. (118, 148)
-Half-and-half--two turning motors, one per side/end (depending on setup). (111)

Then you get into some more interesting setups. Team 1565 had a cool one this year--it's not a true swerve, but it almost acts like one set up to only go at right angles when turning.