A sub-domain is a site within a domain. Subdomains? are always subdomain.domain.tld. In the case of this site, the subdomain is site, and the domain is fsp.im. Subdomains are good for creating unique websites as they don’t require the charge of another domain. The only issue is the hosting company setting limits on the number of subdomains. If I had my own hosting company, there will be no such restrictions.