Service dogs perform a service, like guide dogs for the blind or dogs that are trained to assist a wheelchair bound person by retrieving specific items etc. A therapy dog is no different than a pet, only in having it registered as such allows a person to claim their pet provides them with emotional support, usually for mental health issues such as depression, PTSD etc. Anyone can request their pet be considered a therapy animal, this them allows them access to certain housing just as a service dog is allowed access to public places where pets would normally not be allowed.