Folding @ Home is a crowd source supercomputer program through stanford. You use your computer's processor/graphics card to do a small part of a larger protein folding simulation, and when everyone else finishes their parts researchers at stanford can analyze the data to further research into cancer, alzheimers, etc. You just install the program, tell it how much processing power it can have, and let it go.
"ppd" is points per day, or a measure of how much computing you're contributing per day