A sub-domain is a site within a domain. Sub-Domains are always sub-domain.domain.tld. In the case of this site, the sub-domain is host, and the domain is fsp.im. Sub-domains 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 sub-domains. If I had my own hosting company, there will be no such restrictions.