Microsoft ASP.NET AJAX includes a complete multi-layered client-event model. The Sys.Application class provides events at the application level. The Sys.WebForms.PageRequestManager class provides events that pertain to parts of the page involved in partial-page rendering. Individual components, such as controls and behaviors, have their own events. For more information about these events, see ASP.NET AJAX Client Life-Cycle Events.

ASP.NET AJAX also enables you to add events to the client life cycle. The Sys.UI.DomEvent class enables you to bind HTML Document Object Model (DOM) events to custom ASP.NET AJAX components. In addition, the Sys.EventHandlerList class lets you create new ASP.NET AJAX client events directly.

ref: http://asp.net/AJAX/Documentation/Live/tutorials/CustomClientEvents/default.aspx