View State is the maintain the controls properties Under postback implimentaion that is called view stae.
View State deos not allocated any memory on client side and server side.
view state is maintain the with in the page only.
view state stired the values in hidden fields.
view state is maintain Application level,page level etc.
Session:Session is maintain the User Information on server side .
Session can be  maintain the each client only,but not more than one client.
By using session you can pass the values from one page to anther page .
Session disadvantages is the allocated the More Memory in Server side

ViewState allows the state of objects to be stored in a hidden field on the page. ViewState is transported to the client and back to the server, and is not stored on the server or any other external source. ViewState is maily used the retain the state of server-side objects between postabacks. while working with classic asp we used to have hidden variable to maintain the state of the object.

Programmatically, session state is nothing more than memory in the shape of a dictionary or hash table, e.g. key-value pairs, which can be set and read for the duration of a user's session. For example, a user selects stocks to track and the Web application can store these values in the user's ASP session instance:

http://msdn.microsoft.com/en-us/library/ms972429.aspx