When you have a complex control, like DataGrid, writing an
event processing routine for each object (cell, button, row,
etc.) is quite tedious. The controls can bubble up their
eventhandlers, allowing the main DataGrid event handler to
take care of its constituents.

.Net Framework provides a technique called event bubbling 
that allows a child control to propagate events up its 
containment hierarchy.Event bubbling is used by the data-
bound controls (Repeater, DataList, and DataGrid) to expose 
command events raised by child controls (within item 
templates) as top-level events.A control can participate in 
event bubbling through two methods that it inherits from 
the base class System.Web.UI.Control. These methods are 
OnBubbleEvent and RaiseBubbleEvent.

some complex control like datagrid ,repetator control have 
other child controls inside them.These child control donot 
raise there event by them selves rather they pass the event 
to the container parent . This is known as bubbled event.

Bubbled Event is server side control, Like datagrid,data 
repator,GridView Control

when the child control events are handeled by parent 
control it is called as event bubbling.
for ex. when a control in the datagrid is clicked it is 
handeled by the dtagrid event and not by the control itself 
which fired it so that time it is called as event bubbling.