What is Fragment Cache? Who do we need it?

Fragment Caching :
      Its used to cach the particular portion from a web page. If we have four user controls on a web page we do not want to cache the entire page instance of we need to cache only two controls from a web page. In this case we should go for fragment caching. It caches when the user control loaded at runtime.
Example :
      We need to create two user control to test this fragment caching. In a first .ascx page we should place a label control and also specify assing date and time in page_load.
In the second .ascx page same thing we need to do and we need to place radiobuttonlist control and  command buttong to refresh the page. In the top of the second .ascx page should include outputcache directive like this
<%@OutputCache VarByParam="none" VarByControl="rdbuttonid" Duration="3" %>
This is very useful for cach the particular part from the page.