Have a look at
How to write your own partial postback in ASP.NET 2.0[
^].
Excerpt -> "At the heart of the partial post back construction is the XMLHttpRequest, a DOM API. It can be used inside a web browser scripting language, such as JavaScript, to send an HTTP request directly to a web server without having to reload the entire page and handling the response from the server again within the scripting language. This data, in the form of XML, can then be used to manipulate the page elements on the client side.
On the server side, we implement an HttpHandler to handle the request and pass back data in a valid XML form.
By doing so, we are preventing page refreshes and roundtrips of static data and content in the web pages"
This indicates that a partial postback occurs when you are refreshing a part of the page using some mechanism.
A full postback would be when you refresh the entire page.