I like to give some example.

1 server with one IP address and registered for .com domain is hosting aaa.com , bbb.com or ccc.ddd.com. In some web sites we see email address like below aaa@bbb.com<fff@ggg.com> how is that possible. I cand understand how DNS is resolution takes place. Secondly what is virtual host.

It's all done server side. The domain is assigned a folder within the server. So when someone looks up the web address, the DNS servers kick the name over to the IP that is affixed to the server. The server picks up the name, matches it to the folder, and opens the index file for you to view.

A virtual host is exactly what it's name implies. It is a "virtual" server. Meaning the whole server is not dedicated solely to hosting whatever content a person may put on the server. For example, there are these programs called WHM (Web Host Managers). They set up file structures that mimic a normal file structure on a server. The computer is told to believe that each folder is considered a server and to treat it as such. That way, the multiple folders never interact with each other, and they appear to be it's own server. The owner of the folder couldn't move outside his own folder because the computer would tell him he's at the root folder of his "server".

One PC may have more than one IP. So it may have more than on hostname. For each Network Card you can give different IP Address.