Yes, if you call Session.Abandon() and redirect the user, the will be logged out. This ONLY applies if you are using session variables to determine if a user is logged in if, however, you are using cookies you will need to write code that will terminate the cookie as Session.Abandon() has no effect on them.
What I mean by dropping all session variables is that once you call Session.Abandon() and session variable that you may have created for the current user will be gone.
Think of an entire session as an array and everytime you create a Session Variable, you add a dimension to that array that holds some information. When you call Session.Abandon() our hypothectical array will have all of its dimensions removed thus any session data associated with the current session is deleted.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^
Technical Editor for: Professional Search Engine Optimization with ASP.NET
Why can't Programmers, program??