Definitions for ASP.NET Viewstate

ASP.NET view state is the technique used by an ASP.NET Web page to persist changes to the state of a Web Form across postbacks

From:

http://msdn.microsoft.com/en-us/library/ms972976.aspx

ASP.NET page framework uses to preserve page and control values between round trips

http://msdn.microsoft.com/en-us/library/bb386448.aspx