to follow up on guis post and further explain crab drive....
as gui stated your wheels essentially have a 360 degree rotation. lets say you have a robot that has normal wheels...
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this robot can only move in a front to back motion much like a car.
a robot that has crab drive would have 'crab modules'. basically separate wheel modules which turn about an axis that is perpendicular to the axis of the wheel itself (in other words vertical). the wheel module would be rotated about this axis. the amount of rotation would be measured by a potentiometer that is mounted on top of this crab module (connected to the vertical shaft).
this said our robot that could originally only travel in a forward or backward movement can now move side to side or diagonally according to how far the wheel is rotated about the vertical axis. this in effect gives our robot the ability to move like a crab (crabs in real life normally move in a sideways motion). hence the name 'crab drive'.
(crab modules turning 180 degrees)
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if you would like to learn more feel free to stop by team 111 in the pits at nationals. and maybe if this programmers meeting happens we can try to explain some of the coding behind the way it works. (as i stated before there are many many different ways to go about it and i would love to see how other teams go about attaining this goal)
heres a short clip of our robot using crab drive
http://www.wildstang.com/2002/video/crab.mov