It is 2 omni-wheels, 2 traction wheels.
This means when it isn't swerving, it skid-steers smoothly.
The metal plates on top of the gears are used with a washer as part of a custom thrust bearing that the swerve module rotates on. This results in a high amount of module stability, while minimizing rotational friction.
For more pictures, and some videos:
http://www.vexlabs.com/video/swerve/