WVO stands for Waste Vegetable Oil. It can be used in place of diesel fuel with some modifications to the vehicle.  Generally, WVO needs to be heated before it can be used as a fuel.  Modifications to your car usually include an additional tank for the WVO or some way to preheat the oil before it reaches the engine.  Additionally, the oil must be filtered before you put it in your tank.  Most WVO users feel that the best level of filtration is down to 5 microns or better.

For more information, see one of the following sites:

Good Grease
Frybrid
Greasecar
Golden Fuel Systems (Greasel)