View State in Salesforce

In Visualforce, page state is persisted as a hidden form field that is automatically inserted into a form when the page gets generated. We call this the view state of the page. The view state captures the state of the page -- state of its associated controllers and extensions and the component tree on the page. The view state is posted back along with the other form data, which gives the server enough information to recreate the page state to which new changes can be applied.
Maximum view state size in a Visualforce page : 135 KB