View state refers to the page-level state management mechanism, utilized by the HTML pages emitted by ASP.NET applications to maintain the state of the web form controls and widgets. The state of the controls is encoded and sent to the server at every form submission in a hidden field known as __VIEWSTATE. The server sends back the variable so that when the page is re-rendered, the controls render at their last state. At the server side, the application may change the viewstate, if the processing requires a change of state of any control. The states of individual controls are decoded at the server, and are available for use in ASP.NET pages using the ViewState collection.
The main use for this is to preserve form information across postbacks. ASP.NET View state is turned on by default and normally serializes the data in every control on the page regardless of whether it is actually used during a postback. This behavior can be modified, however, as View state can be disabled on a per-control, per-page, or server-wide basis.