While a domain is a full website address, totally independent of other domains, a sub domain depends on a domain. If for example “yourdomain.com” is your domain, then http://yourdomain.com and http://www.yourdomain.com would be your domain name’s address.
A sub domain name would be a name *under* your domain name, something like “subdomain.yourdomain.com”. Its address would then be: http://subdomain.yourdomain.com and http://www.subdomain.yourdomain.com. It can say anything you want instead of “sub domain” and the nice thing is that unlike a full domain name, a sub domain does not need to be registered. You can add it for free.