Hi Katherine,
By default users login using their email address. There is a setting in Site Settings for "Use Email For Sign In". It is checked by default, but if it were unchecked then the user would have to login using the user id instead of email address. Be careful if you change that then you need to sign out and sign in again because the authentication cookie is set differently depending on whether email or user id is used (make sure you know your user id or you might lock yourself out).
What may be confusing is that we label this as user id in the my account page, but it actually maps to the database mp_Users table to the LoginName field. And there actually is a field on the table named UserID which is an integer but that is not what we display as user id in the my account page. So what we are calling user id in the web ui is really the login name. After registration this login name is never used unless you were to uncheck the "Use Email For Sign In" setting in site settings, then it would be needed instead of email address when logging into the site.
Hope that helps,
Joe