Control Execution Lifecycle

The server loads an ASP.NET page every time it is requested and then unloads it after the request is completed. The page and the server controls it contains are responsible for executing the request and rendering HTML back to the client. Although the communication between the client and the server is stateless and disconnected, the client experience must appear to be that of a continuously executing process.
This illusion of continuity is created by the ASP.NET page framework and by the page and its controls. On postback, a control must behave as if it were starting where it left off at the end of the previous Web request. The ASP.NET page framework makes it relatively easy to perform state management, but control developers must be aware of the control execution sequence to achieve the effect of continuity. Control developers need to understand which information is available to a control at each phase in its lifecycle, which data is persisted, and what the control's state is when it is rendered. For example, a control is unable to invoke its parent until the tree of controls on a page has been populated.

http://msdn.microsoft.com/en-us/library/aa719775(VS.71).aspx