about IsPostback method

Post back is the process the sending data get back to server
for processing.
Is post back property checks whether page is being loaded for
first time.
if it is False :means first time loaded.
if it is True: means round trip to server.

Gets a value that indicates whether the page is being rendered for the first time or is being loaded in response to a postback.

http://msdn.microsoft.com/en-us/library/system.web.ui.page.ispostback%28v=vs.110%29.aspx