Hai Srinivas Kotha,
Automatic persistent of form data during the post back implementation is called as View State.
As the Web is stateless so it doesn't recognize anything when any post-back happens. each time a new page gets created with all it's events.
Soto retain the original content, we have the feature called EnableViewState at the page level in which we an retain the original content of the page and regenerate the whole page and populate the same content.
Internally it gets the check-sum of all the controls and then create a BASE 64 encrypted content and place in the same page so that when the page gets loaded again after the post back, again it check and based on the content renders the page contents.
Hope it will be helpful to you.
Pawan Awasthi(DNS MVM)
+91 8123489140 (whatsApp), +60 14365 1476(Malaysia)