a. With the help of delegate control, we can take any OOB control of SharePoint and replace with our custom control without any modification in the SharePoint page. So that new custom control overrides the existing one.
b. So the delegate control provide one of the option to add control (either server control or user control) on a SharePoint page.
c. This is one of the important features of SharePoint, introduced in WSS v3 and so in MOSS 2007.
d. For example : In master page SearchBox control is included as
<SharePoint:DelegateControl runat="server" ControlId="SmallSearchInputBox" />
e. The above lines instantiate the delegate control object.
f. This delegate control object uses features to locate the control which is specified in ControlId. So there must be the features (it includes feature at any scope) which creates and deploys the SmallSearchInputBox control.
Thanks and Regards