The Americans with Disability Act prohibits discrimination against people with disabilities in several areas, including employment, transportation, public accommodations, communications and access to state and local government’ programs and services.
Individuals with disabilities may use service animals for a variety of reasons.
II. Service Animal Defined by Title II and Title III of the ADA
A 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. Tasks performed can include, among other things, pulling a wheelchair, retrieving dropped items, alerting a person to a sound, reminding a person to take medication, or pressing an elevator button.
Emotional support animals, comfort animals, and therapy dogs are not service animals under Title II and Title III of the ADA. Other species of animals, whether wild or domestic, trained or untrained, are not considered service animals either. The work or tasks performed by a service animal must be directly related to the individual’s disability. It does not matter if a person has a note from a doctor that states that the person has a disability and needs to have the animal for emotional support. A doctor’s letter does not turn an animal into a service animal.
More info on the ADA can be found here