Diffrence Between session.clear() and session.abandon()

Session.Abandon() destroys the session and the Session_OnEnd event is triggered.
Session.Clear() just removes all values (content) from the Object. The session with the same key is still alive.
So, if you use Session.Abandon(), you lose that specific session and the user will get a new session key. You could use it for example when the user logs out.
Use Session.Clear(), if you want that the user remaining in the same session (if you don't want him to relogin for example) and reset all his session specific data.