Q:

Why we use jQuery selector with colon? 

I was recently going through a jquery code where they had used 'div:#idname' as selector.  What is the use of this selector with colon? 
here is the link to js file
http://www.amitpatil.me/demos/twitter-like-topwtweets/js/toptweet.jquery.js

A:

There is none in that example; it's an invalid selector. 
The colon is used to introduce various pseudo-classes like state pseudo-classes (:hover, :active, etc.),  structural ones (:nth-child, etc.).  Two colons are used to introduce pseudo elements like ::before and such, but div:#idname is invalid.