Folding@Home helps, simply said, researches to get more understanding of complex biological processes. This knowledge can be used to develop medicines that can cure diseases like Alzheimer's.
Since the end of September 2004, there is Team Firefox: a Folding@Home team consisting of Firefox users and supporters. Everyone is allowed to join and help spread Firefox and help researchers to get better understanding about diseases (and maybe even find cures to them). More information about the team (including how to join) is on the Spread the Fox homepage.
Why Folding@Home instead of other distributed computing projects? Because Folding@Home has produced more results than other projects. It can at the end help save lives, by finding cures to diseases. It might actually save your life later, in contrast to other distributed computing projects that check if there are aliens, crack mathematical algorithms or forecast what the climate will be when you've passed away for a long time already (maybe because of a disease which Folding@Home could provide cures to).
Folding@Home is Stanford University's distributed computing project which studies protein folding, misfolding, aggregation, and related diseases. It uses novel computational methods and large scale distributed computing, to simulate timescales thousands to millions of times longer than previously achieved. It allows simulating folding for the first time and examine folding related disease.
What are proteins and why do they fold?Edit
Proteins are biology's workhorses -- its "nanomachines." Before proteins can carry out their biochemical function, they remarkably assemble themselves, or "fold." The process of protein folding, while critical and fundamental to virtually all of biology, remains a mystery. Moreover, perhaps not surprisingly, when proteins do not fold correctly (i.e. "misfold"), there can be serious effects, including many well known diseases, such as Alzheimer's, Mad Cow (BSE), CJD, ALS, Huntington's, and Parkinson's disease.