Second-level domains can be divided into further domain levels. These subdomains sometimes represent different computer servers within different departments.The Third-level domain name includes the Second-level domain name. For example, in: sales.hostNdomain.com, "sales" is a third-level domain. "hostNdomain.com" is a second-level domain name (and includes the top-level domain name of ".com").
A sub domain is a sub-division of a larger domain. For example, "example.pixbytes.co.uk" is a sub domain of "pixbytes.co.uk."
Are prefixes to your domain name. (i.e subdomain.domain.com). They are useful for replacing really long web addresses (i.e. http://www.domain.com/main/images/pictures/2005/october) and for hiding locations (i.e. private.domain.com goes to http://www.domain.com/djklszxcnmv/).
Sub-domains allow you to create second-level domain names, taking the form of subdomain.yourdomain.com. Sub-domains are useful to create sub-sections of your website.
A sub domain, also known as a "third-level" domain, is a great way to create memorable web addresses for various areas of your web site. For instance, Yahoo! Uses sub domains such as mail.yahoo.com and music.yahoo.com to distinguish its services. Yahoo! Web Hosting makes it easy to create sub domains.