What is a Sub Domain
Sub-domains are a way to partition a <domain name> into smaller areas. If your web site is your home on the internet, then sub-domains are rooms in your home. Each sub-domain can act as a completely separate site, or as a seamless part of the larger site.
Recognizing sub-domains
Sub-domains are easily recognizable by their different prefix. For example, About.com is a well known domain name that concentrates information about many topics. Though the site seems all of one piece, each top-level topic lives in its own sub-domain. Thus, for example, you'll find all information regarding action figures in the actionfigures.about.com sub-domain (notice the change from "www" to "actionfigures").
Creating sub-domains in your site
Sub-domains are a service offered by some hosting providers in some of their plans. Some plans may offer you a fixed number of sub-domains under your main domain names; others may offer an unlimited number. If you plan to employ sub-domain, make sure you choose a plan that supports your needs.
You will likely be able to manage your sub-domains yourself, through your online site dashboard. When you set up a new sub-domain, you have the option to associate it with a certain folder or directory in your site. That directory will serve as a self-sufficient Internet site for the sub-domain, and will contain its own index page, web pages, etc..
Another option is to point your new sub-domain to the same directory as your general site. Take care when choosing this method, since URLs may act unexpectedly: unless you have some control mechanism in place, you will be able to reach all pages through either address (the regular domain and the sub-domain), which might seem to search engines as duplicate content, and might get you demoted or even blacklisted on some.