vortex generators produce basically, small, horizontal(usually) tornadoes(vortecies).  Faster moving air usually produces lower pressure than slower moving air(look at how a wing creates lift).  In a high performance racecar you want lower pressure under the car and higher pressure above the car(downforce). The vortex generators are positioned so that the vortecies will form in an area where faster moving air (lower pressure) is desired.  Vortecies are beneficial but not always implemented in; diffusers, (underbody tunnels), dual element wing slots, dive planes, underneath wings, and NACA inlet vents.

so basically, a small whirl of high speed spinning air is formed under the car or a part of the car such as a wing. This high speed creates lower pressure, and the ensuing
downforce. :)

P.S two large problems with vortex generators is that they creat turbulent non linear airflow that creates drag. Also I believe that they only begin creating a vortex at a certian speed. would anyone like to confirm or deny this?

clueless2,
I have learned a great deal about this from Gary Wheelers website.  The example he shows is the use of vortex generators on the bottom side of the rear wind on race cars, indy, and top fuel dragsters.  Vortex generators help to prevent the flow separtion so air car wrap around a corner at high speed by creating tiny tornados that can keep airflow on the surface.  They do creat turbulent non linear flow that crates drag in a straight on headwind, but I have found these to be benificial in crosswind conditions to shed the air and streamline large vehicles reducing aerodynamic drag significantly.  Check out the followins site, and on it is a link to wheelers site.  http://airtabs.com