If you allow it in the configuration, users can register themselves when they click on "New User" in the "Login" page.
They fill out the email and then get a password sent to them by Email. This is to verify that the Email is valid.
New users have no special rights, unless you add them to the configuration file.
Passwords are not saved in clear, but as a a hash. If the user has lost the password, a new one is be sent to him with an email.
The easiest way to create a new user is to go to the Special:Passwords page. Then fill in email and password. If you want to copy the rights of another user, add it in the field User Template. The special page then creates a draft for a new user page you can preview and then modify.
Note that you can also grant rights to specific machines as IP users without the need of a login.
Developer can have multiple users without login, see Alt users.