A Therapy Dog primarily goes to various facilities, such as schools, hospitals, nursing homes, etc. and offers comfort to others. Therapy Dogs are certified by an independent organization which provides liability insurance for the dog. Therapy Dogs must ask permission to go into a public place, and are not allowed in most stores.
A Service Dog serves to alleviate his owner’s a disabling by performing tasks that are trained. Service Dogs Handlers may bring their trained service dog with them almost everywhere they go. You will see from time to time, website selling “Service Dog Certification.” This is a scam. There is no legal requirement for certification of Service Dogs. Also, anyone is allowed to train a Service Dog and that leads to another common scam, selling untrained or poorly trained dogs to unsuspecting disabled persons.
Emotional Support Dogs are pets that provide comfort to their owners and only allowed in pet friendly places. They don’t require any special training and support their owners with affection. However, with proper documentation, ESA’s are allowed on some planes (with permission) and in housing which otherwise prohibits pets. An emotional support animal is not necessarily a dog, but any pet that supports you, such as a cat or bird.