Once you sign up for an account, you can list up to 3 characters per a screen name and start keeping track of which mounts you have and which mounts you want to have. You can also ignore the mounts you don't want or can't get. Once you have it all sorted out, you can share your collection with others if you wish (For example, here's Mania's collection).
The site has always been a pretty handy tool for mount collectors or people who just want a refresher about what forms of locomotion are out there, and the collection tool only makes it better. Kudos to Mania, who's once again come through with a nice little tool for the WoW community.