Here's a little overview of how it works -- but basically, it's a chance to give away your used books to people who really want them, and get books you want in return, for nothing more than the cost of postage. The sender pays, the receiver doesn't; but you have to keep up your sent-to-received ratio or you can't mooch any more books, so it works for everyone.
I could never quite figure the point of LibraryThing, but I can definitely see the point of this. Anyway, the more people involved the better the pickings, so -- sign up! Share books! Let me mooch off you! :)
P.S. to jamesbow: Panic not. I bought two copies of Fathom Five at your launch -- you can bet I'm hanging on tight to the other one. And somebody's already claimed the extra, so go you!