Control State is another state management technique which is essentially a private view state for your control.
In order to preserve the page and control data over round trips to the web server, traditionally we are using View State. View State Property maintains information in hidden field and the data is encrypted.
If you disable the View State in your page, the custom controls which used in your page has a chance of losing their state.
To Address the above issue ASP.NET 2.0 has introduced Control State which is private View State for the control and preserves the state of the control even when you turn off the View State.
Use the Control State property when you are developing the Custom Control by overriding the SaveControlState method to save the data in Control State.
Note that Control State Can not be disabled.