By design HTTP, the protocol used to request and deliver web pages, is stateless. What that means is that the server does not keep track of requests you made to the browser. As far as the server is concerned, each request for a page is an entirely new one and is not related to any previous request you may have made.
This of course causes issues if you need to maintain data for a specific user. To overcome these problems, web developers have a number of solutions available, including session state, cookies and hidden form fields. ASP.NET Web Forms has been hiding much of this complexity by implementing a concept called View State. Controls (including the Page class itself) can read from and write to the View State collection to maintain data across postbacks. Controls such as Label use this mechanism to send their values to and from the browser, maintaining them across postbacks.
In this article you'll see how to leverage View State to store your own values as well.