Yes sandrabusby.hubpages.com is a subdomain of the domain of Hubpages.com
I am not sure why you ask but to break it down:
A domain is an area or territory controlled by a government and in the web world it is a corner that is controlled by a particular owner. A domain requires a domain name that can be solely owned. There cannot be two of the same names so the domain and names are owned by Hubpages and it owners. The subdomain is a part of the larger domain as stated by the DNS system.