what is the difference between viewstate and control state?

This example shows how to create a custom control named IndexButton that uses control state to maintain critical state information across page requests. Control state, introduced in ASP.NET version 2.0, is similar to view state but functionally independent of view state. A page developer can disable view state for the page or for an individual control for performance. However, control state cannot be disabled. Control state is designed for storing a control's essential data (such as a pager control's page number) that must be available on postback to enable the control to function even when view state has been disabled. By default, the ASP.NET page framework stores control state in the page in the same hidden element in which it stores view state. Even if view state is disabled, or when state is managed using Session, control state travels to the client and back to the server in the page. On postback, ASP.NET deserializes the contents of the hidden element and loads control state into each control that is registered for control state.

Use control state only for small amounts of critical data that are essential for the control across postbacks. Do not use control state as an alternative to view state.