It basically does two things: it allows you to catalog all your books in a sharable format online and it's a community website. As you enter books, it automatically finds other people with your books, gives you suggestions about books of the same topic that other people have enjoyed, shows you ratings and reviews, offers (brand new) discussion groups on various topics and so on and so forth. The more books you add, the better the site gets.
Entry is stunningly simple. It can look up names, ISBN, LC numbers, whatever you have from your books in a variety of sources and find then and pull them down. Most of my books are findable in Amazon, but a few of my rare books were found in the Library of Congress. It's pretty fast. You add them, go back, tag them, and then it enters your tag into a "cloud" of tags on that book. You can then slice and dice your books however you want.
I've only entered 191 books so far -- I have not yet gotten up to the library upstairs or entered in the art books or the cookbooks, so this is only a sliver -- but if you want to see an example, here is my catalog.
As a bonus, it has instructions on how to import all your books from your amazon order history to get you started.