as sharepoint is made from asp.net, I would assume its an intergral part of sharepoint. There are other methods other than session state that you can use depending on your criteria. Session state can also hinder on performance or result in application errors if used inappropriately and will allow you to properly clear the session state.
to better understand session state like ttl (time to live) which i think is 20 min depending on the user not using the session again, security and most of all performance... msdn would be the best place (links below).
sharepoint does use session state, just not the form you know of ;)
Session State Providers
for the understanding and all your answers on session state and sharepoint:
to create a session state:
How to save variables per user session per page
the above link is my answer to using session state! its simple and easy and reusable :)
about asp.net session states