From the Americans With Disabilities Act (the ADA is the law that gives disabled people with service dogs access to public accommodations like parks, hotels, stores, restaurants, etc.):
Service animal means any dog that is individually trained to do work or perform tasks for the benefit of an individual with a disability, including a physical, sensory, psychiatric, intellectual, or other mental disability. Other species of animals, whether wild or domestic, trained or untrained, are not service animals for the purposes of this definition. The work or tasks performed by a service animal must be directly related to the handler´s disability. Examples of work or tasks include, but are not limited to, assisting individuals who are blind or have low vision with navigation and other tasks, alerting individuals who are deaf or hard of hearing to the presence of people or sounds, providing non-violent protection or rescue work, pulling a wheelchair, assisting an individual during a seizure, alerting individuals to the presence of allergens, retrieving items such as medicine or the telephone, providing physical support and assistance with balance and stability to individuals with mobility disabilities, and helping persons with psychiatric and neurological disabilities by preventing or interrupting impulsive or destructive behaviors. The crime deterrent effects of an animal´s presence and the provision of emotional support, well-being, comfort, or companionship do not constitute work or tasks for the purposes of this definition.
From the US Department of Housing and Urban Development's Guidebook for Practitioners (HUD enforces the Fair Housing Act which prohibits discrimination against the disabled in housing matters and gives the right to emotional support animals in no-pets housing):
Assistance animals include guide dogs for persons with visual disabilities, as well as other types of assistance animals that provide services for people with other disabilities, including emotional support for persons with mental disabilities or whose disabilities result in chronic pain.