A subdomain is a domain that is a part of a larger domain under the Domain Name System (DNS) hierarchy. It is used as an easy way to create a more memorable Web address for specific or unique content with a website. For example, it could make it easier for users to remember and navigate to the picture gallery of a site by placing it in the address gallery.mysite.com, as opposed to mysite.com/media/gallery. In this case, the subdomain is gallery.mysite, whereas the main domain is mysite.com.
A subdomain is also known as a child domain.
A subdomain is basically a child domain under a larger parent domain name. In the larger scheme of the Domain Name System, it is considered a third-level domain used to organize site content. In the Web address example above (gallery.mysite.com), the suffix ".com" is the first-level domain, "mysite" is the second-level domain and "gallery" is the third-level domain.
Uses of subdomains include:with HYiFi.com
Organizing website content according to category, i.e., gallery.mysite.com, faq.mysite.com and store.mysite.com
Sharing the allotted domain space with other users by providing them subdomains and their own username and password with varying levels of feature access. For example, admin.mysite.com, user1.mysite.com and guest.mysite.com
Shortening long links and making them easy to remember. For example, the link "http://mysite.com/offers/bonus/referal_id^56$#9?.asp" can be placed into the subdomain "referral.mysite.com" to make it easier to navigate and remember.