If a user is removed from the Site Admin the users name will stay on the test, defect, etc. The test, defect, etc will continue to be accessable. The same goes for removing users from a project. The entities associated with that user will continue to function and that users name will stay on the entity until changed.
The difference between a disabled user and a deleted user is that when you disable a user all the security information is retained (ex. project groups) and the user stays in the site admin database. When you delete a users all of that users information is removed from the site admin database.