An "active user" is any user in your LMS who can sign in concurrently. For example, "25 active users" means that only 25 users can login to your LMS instance at the same time. Logged in users who are idle for an hour will be logged out and removed from the active users count. You can also manually logoff users in 'Account & Settings' --> 'List users' page under user options.
A "Registered user" is any user who has an user account in your LMS instance. You can have unlimited registered users created.
Any user logging in to the LMS with a named user account is counted toward "Active users" limit. You can invite users by e-mail address option to a live virtual classroom session and these users are not counted towards the active users limit. They can join a live session by simply clicking on the secure link sent in the email invite without signing in to the LMS. Virtual Classroom limitations will apply in this scenario.