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Description:
The AJAX Timer control enables a portion of an ASP.NET web page to be dynamically updated at a regular interval, rather than needing the user to perform an action such as clicking on a button.

Timer control performs a postback at a specified time; on postback the Timer's Tick event is raised. If the Timer control is placed within an UpdatePanel it triggers a partial page postback, during which we can rebind other controls to update their display[1];

Notes:
The Timer control has three client-side methods that we can use to achieve this desired functionality[1]:

• _startTimer() - starts the Timer control
• _raiseTick() - causes the Timer control to "tick," thereby posting back and raising its Tick event on the server
• _stopTimer() - stops the Timer control

Related articles:
[1] - Master Pages and ASP.NET AJAX: the Timer control is used to refresh other controls on the page. It also explains how to interact with the Timer control from client-side script (JavaScript);
[2] - Timer control : a small example of using the Timer control. It simply updates a timestamp every 5 seconds;
[3] - How Do I: Use the ASP.NET AJAX Timer Control?: video tutorial;