Question : DNS delegation

I would like to know what exactly is DNS delegation & what is the purpose of it.
Where it is exactly used.?

Answer : DNS delegation

The following link shows how to delegate subdomains.  Is that what you are asking about?   For example you own xyz.com.  You would control host names like www.xyz.com, ftp.xyz.com, etc. You'd like to give permission of someone else to control abc.xyz.com, so they could control www.abc.xyz.com, etc.

http://www.zytrax.com/books/dns/ch9/delegate.html

It is used to give someone else control over part of your domain.  ISP's use it to allow their clients to get control over their domains as well, or their reverse dns lookups.