From Misc Data Digs
Logging in is not required for viewing pages, and not even for editing them. However, it provides additional features, and in general many recommend it.
Creating a user account means that you supply a username (your real name or a nickname) and a password. The system will reject a username that is already in use. A user account is created only once. You are then "logged in". Next time you log in you supply your username again and demonstrate with the password that you are the same person.
Edits you make are recorded under your username. If you are not logged in your edits are recorded under your IP address and your IP shows up on recent changes list, on people's watchlists' and many other places.