What it is is a website where you can catalog your book collection, view the libraries of others, get suggestions as to what else you'd probably enjoy, get suggestions as to what you probably wouldn't enjoy, and so on . . .
I've had a similar utility on my mac called Delicious Library, which I actually prefer in some ways, but the online component of LibraryThing cannot be overstated. It makes cataloging my library feel a bit like a game to me, honestly, like my goal is to have more books than anyone else, or a more complete catalog of my books, or better recommendations or something.
I guess my only warning would be that if you're incapable of reading, you probably wouldn't enjoy it. Otherwise, it's pretty cool.
I'm on there as "Myca."