From the ADA brief 2010 http://www.ada.gov/service_animals_2010.htm
" Service animals are defined as dogs that are individually trained to do work or perform tasks for people with disabilities. Examples of such work or tasks include guiding people who are blind, alerting people who are deaf, pulling a wheelchair, alerting and protecting a person who is having a seizure, reminding a person with mental illness to take prescribed medications, calming a person with Post Traumatic Stress Disorder (PTSD) during an anxiety attack, or performing other duties. Service animals are working animals, not pets. The work or task a dog has been trained to provide must be directly related to the person’s disability. Dogs whose sole function is to provide comfort or emotional support do not qualify as service animals under the ADA."
This definition does not affect or limit the broader definition of “assistance animal” under the Fair Housing Act or the broader definition of “service animal” under the Air Carrier Access Act.
Some State and local laws also define service animal more broadly than the ADA does. Information about such laws can be obtained from the State attorney general’s office."
Service Dogs come in all different sizes and breeds. They must have the right temperament in order to handle a variety of environments, people and situations. They must also be task trained to aid their disabled handler, under the ADA they must do more than one task or work (emotional support or comfort are not tasks). Whether owner trained or going through an organization, service dog training takes about 18mo - 2years. Service Dog Handlers have public access rights under the ADA which means that their service dog can go in most public places (some exceptions are sterile hospital situations, commercial kitchens, or where the general public is not allowed)
ESA (Emotional Support Animal)
An ESA is a pet that provides comfort and emotional support to their owner. It has no special training and is usually prescribed by a doctor (you still must be disabled or elderly to have an ESA). ESAs can help people with PTSD, Anxiety, Depression, Alzheimers ect. ESAs can live in non pet housing and fly with their owners the two areas of law regulations that pertain to them are HUD Fair Housing Act and FAA Air Carrier Access Act (both can request doctors letters). The ADA does not cover ESAs in public access and is very clear that providing comfort and emotional support only are NOT tasks.
Therapy Dogs are not a type of service dog, they are not trained to aid their handler but instead to help multiple people. They are a type of working dog with a great temperament and usually have basic obedience and have to pass a test to be registered with a therapy dog organization. Most places will require that a therapy dog be registered with an organization due to insurance and liability coverage. Therapy dogs provide comfort and affection to people in hospitals, nursing homes, after disasters happen and even help students destress during finals.
There are no federal laws that require a service dog or esa to be registered or have ID. The ADA even states "Staff cannot ask about the person’s disability, require medical documentation, require a special identification card or training documentation for the dog, or ask that the dog demonstrate its ability to perform the work or task." There are several sites online that offer "official" registrations, certificates and IDs these are all scams to make money off disabled people and people who want to bring their pets everywhere. Most of these registry sites will also state wrong info about how you can take your ESA or Pet everywhere with you in public places (refer to the ADA link above for why this is wrong). It is against the law and even punishable in some states to present your dog as a service dog when it is only a pet.
There is nothing "official" you do to make your dog a service dog except to follow the ADA:
1) have a disability (the full ADA has a definition of disability)
2) Your dog is trained to do tasks that mitigate that disability
3) your dog is behaved in public and not be aggressive or destructive in public