You can use it to search for fanart - every image is tagged. It's also joinable, so if you register an account (no email address required) you can upload images and edit existing tags to make it easier to search.
The only real rules are not to violate the Booru Project TOS (basically no loli/shota, really) and a general note that providing source is better than not providing source, if you can.
Hopefully this should be useful to people hoping to find specific pictures, or looking for art of a certain pairing, or.. anything like that. ♥