When you log in to a Wikia site, you also log in to other Wikia projects, a process called Unified Login.
If you create an account on any Wikia site, Wikia will automatically create accounts for you on all other projects as well (except for Uncyclopedia). Usually wiki policy is to welcome the user after they have made their first edit, but don't be alarmed if you get messages on other wiki sites.
Will Unified Login log me on to wikis outside of Wikia?Edit
Unfortunately not. If you wish to join a wiki outside of Wikia, you will still have to create an account there.