view state that automatically preserves property values of 
the page and all the controls on it between round trips.

Actully All HTML controls are state-less ie;-these can not 
store any type of clients inputs.when ever we send the page 
to server it will send this page as fresh one.so we loose 
the data what we have entered.

To store all these data (temporarly) in clients browsers ,
.Net provide a concept that is known as View state .View 
state store the data in encoding format,so that one can not 
gues it easily.

view state is used to find the current state of an object,
forexample with the help of view state we can find that 
whether is a button is click or not;

viewstate is just like hidden field.Basically all html
controls are stateless .if u want maintain state for page
level we can use viewstate.

ViewState is a built-in structure for automatically 
retaining values among multiple requests for the same page. 
The view state is internally maintained as a hidden field 
on the page but is hashed, providing greater security than 
developer-implemented hidden fields do.
Following are the benefits of using ViewState:- 
	No server resources are required because state is 
contained in a structure in the page code.
	Simplicity.
	States are retained automatically.
	The values in view state are hashed, compressed, 
and encoded, thus representing a higher state of security 
than hidden fields.
	ViewState is good for caching data in Web farm 
configurations because the data is cached on the client. 
Following are limitation of using ViewState:- 
	Page loading and posting performance decreases when 
large values are stored because 
	View state is stored in the page.
	Although view state stores data in a hashed format, 
it can still be tampered with because it is stored in a 
hidden field on the page. The information in the hidden 
field can also be seen if the page output source is viewed 
directly, creating a potential security risk. 
	If page get destroyed or refresh information which 
is stored in view state gets vanished.

Below is sample of storing values in view state. 
this.ViewState["EnterTime"] = DateTime.Now.ToString();