A subdomain is a domain that is subordinate to another domain. Actually, a second level domain( SLD) could be called a subdomain, since it is subordinate to the top level domain( TLD).
In practice, however, the term subdomain is used for domains that are hierarchically below the SLD domain. Subdomains are therefore also referred to as third level domains.
Domain addresses are read from back to front. For example, at www.e-books.itwissen.info, the top level domain (TLD) is *.info, itwissen is the second level domain (SLD), and e-books is the subdomain or third level domain. Other examples of subdomains are "bilder", "fotos" or "blog". Subdomains provide large companies with the ability to logically organize and separate production operations from sales organizations.
The use of subdomains is also used to divide the content of a domain among several subdomains; namely, to increase the number of active queries. The procedure is called domain sharing.