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/).