Guys, this debate is basically just about terminology. Everyone has their own.
Here is mine (and 229s):
Omni-Wheel - A wheel that rotates normally and functions as a wheel, but can also be moved perpendicular to it's rotation. Like the wheel Patrick posted above. Many teams use these to aid in turning. (Hotbot 2001)
Omni-Drive - A drivetrain that consists of multiple "sets" of omni wheels in different (usually at perp.) directions. This alows the robot to move in any direction simply by powering the different wheels at different speeds. (think Metal-In-Motion 2002, which was a 6 wheeled variety. A 4 wheeled variety can be seen
here .) I would consider 857's kiwi-drive to fall under this category.
Crab Drive - This type of drivetrain has 2 sets of drivetrains set perp. to each other. Instead of using omni-wheels and having all wheels on the ground, it somehow actuates things so that only one set is on the ground at a given time. (Good examples of this include WPI 2003, 358 2003, TRIBE 2002).
Swerve Drive - The individual wheels can "spin" changing the heading of the robot. It can move in any direction. (think Wildstang 2003/2002, Chief Delphi 2001/2002) There are many variations on this.
2-Wheel Swerve- Two wheels "spin" like a normal swerve drive (See above) the other wheels are either casters, or omni-wheels (see definition above). (see Chief Delphi 1998/2000).
Articulated-Steering - Some, or all of the wheels will turn like a car to aid in turning. 2 cool examples of this include Hotbot and Thunderchickens 2003.
Skid-Steering, or Tank Drive - This is the most standard type of drivetrain. All the wheels/tracks/whatever are pointed in one direction. The robot turns by reversing one side, while powering the other side forward. Examples of this include... almost everyone. When a robot in this configuration is turning, some energy is lost to "side load" because the robot is essentially dragging some of it's wheels across the floor sideways. Some teams use dropdown casters/skids to aid in turning.
This is just my terminology for things. It isn't always the most "technically correct" terminology, but this is what I've developed over 4 years in this program. *shrug* it works for us.
Perhaps this will benefit, someone....
John