What is Difference between Callbacks and Postback in ASP.NET?

Callback : It is a way to send a request to the web page
from the client script. Postback is an expensive call with
processing overhead. In callback a client function sends a
request and a special marked method is invoked on the
server. It does the processing & returns the value which is
received by another client function to process the result.
To have client side callbacks, the page has to implement
ICallbackEventHandler & implement functions
RaiseCallBackEvent & GetCallBackResult.

PostBack: Postback is the event which sends the form data to
the server. The server processes the data & sends it back to
the browser. The page goes through its full life cycle & is
rendered on the browser. It can be triggered by using the
server controls.

Callback is also the form of Postback, but the difference 
is in postback execute a complete page life cycle and whole 
page redraw, in callback only specific portion (Contorl) is 
executed/redraw using client side scriptiong.