Subdomain is also known as child domain, which belongs to a part of domain name in Domain Name System hierarchy. It is the third level of a domain name and used to better organize the content of a website, and divide big website into small sections. For example, http://domain.com is the ordinary URL without a subdomain; however, http://posts.domain.com is the URL with a subdomain “posts.”
There are three level of the latter URL. “.com” is the first level or top level of the domain hierarchy; “wordpress” is the second level while “posts” is the third level. Apart from helping to organize a website structure, subdomains also contribute to share disk space with friends by creating subdomains for them and simplify email address. Now, nearly all the web hosts offer unlimited subdomains for users; therefore, website owners can add as many subdomains as they prefer to make their websites tidy.