According to the Americans with Disabilities Act (ADA, 1990), a dog is considered a "service dog" if it has been "individually trained to do work or perform tasks for the benefit of a person with a disability." Also according to the ADA, a 'disability' is a "mental or physical condition which substantially limits a major life activity" such as:
• caring for one's self • performing manual tasks • walking • seeing • hearing • speaking • breathing • learning • working • as well as some disabilities that may not be visible, such as: deafness, epilepsy, and psychiatric conditions
Unfortunately, there is considerable lack of knowledge among the public regarding the rights of the disabled. Many people believe that without a “Service Dog” vest or tag, a dog cannot be a legitimate service animal — and there are several unethical companies that profit by this ignorance. They sell these forms of identification without requiring proper proof of the level of training a dog has had, nor medical documentation of a person’s disability. If a dog does not meet all the requirements listed above, but a person misrepresents their dog as a service animal, they are in violation of a federal law and subject to a heavy fine and/or imprisonment.
The above linked site has informative and clear information on many aspects of service animals.