Subdomains are the third level domains that are used to form your web site content.
They are nothing but just folders in your root directory. But they will have a special URL to access.
http://www.yoursite.com
Is the regular URL without a subdomain?
http://products.yoursite.com
Is an URL with subdomain “products”? Here the:
.com is the first level domain
yoursite is the second level domain
products is the third level domain