[darkpuppet]

folding @ home researches the folding of known proteins (using existing tools to find why they break).
the human proteome (worldgridcommunity) does research on simulating proteins (like building new tools for researchers to use).

the client runs as a low-priority thread on your computer... the clients sit pretty much quiet until they are given processing time. however, the way OSes handle and switch threads, you'll find that you'll want to disable or put to sleep the grid clients when you need the processor's full attention.