A subdomain is a domain within a domain. The subdomain is usually written in front of the domain seperated by a "." for example jobs . professionalhosting.com is the subdomain work of professionalhosting.com, its main advantage is that a subdomain has all the properties of a domain with the difference that this is free and with the only disadvantage that it increases the address slightly.
Our plans of hosting and servers support the creation of subdomains, being able to create even independent ftp for these and resources to measure, all this of simple and intiutiva way.
Note: The most common and known subdomain is "www" Worl Wide Web which means "world wide web", normally all domains are preconfigured with this subdomain.