Omni Wheels are wheels with little rollers around the edge. If you've ever seen a roller hockey puck you know what I'm talking about. The rollers do not reduce friction, the just allow the wheel to have wheels on it so it can move sideways easily. They allow for easy manuevaribility and turning due to the relative massive loss in friction.
So you have a wheel and then all around the rim are much smaller wheels, really rollers, that have a motion range perpendicular to the of a normal wheel, making it easier to move about, but losing traction while your at. I know vex has omni wheels that are super cheap and easy to fool around with, just so you get the concept. Hoep ive been a help
http://www.andymark.biz/plastic-omni.htm