Service animals are animals that have been trained to perform tasks that assist people with disabilities. Service animals may also be referred to as assistance animals, assist animals, support animals, or helper animals depending on the country and the animal's function.Dogs are the most common service animals, although other animals such as monkeys, birds and horses have been documented.Beginning March 15, 2011 only dogs, and in special circumstances mini horses, are permitted to be service animals. http://www.ada.gov/service_animals_2010.htm
The international assistance animal community has categorized three types of assistance animals:
In the United States, the Americans with Disabilities Act (ADA) defines a service animal as "any guide dog, signal dog, or other animal individually trained to provide assistance to an individual with a disability."
As of September 2010, the Civil Rights Division of the U.S. Department of Justice has redefined a "service animal" for the purposes of the ADA as "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." In addition to dogs, the Civil Rights Division carved out an exception to this new rule for miniature horses. Although they are not included in the new definition of "service animal", they are also protected under the ADA under specific circumstances.This revised ADA definition excludes all comfort animals, which are pets that owners keep with them for emotional reasons. (For example, the owner may feel calmer when he or she is near the pet.) Unlike a service animal, a comfort animal is not trained to perform specific, measurable tasks directly related to the person's disability. Common tasks for service animals include flipping light switches, picking up dropped objects, alerting the person to an alarm, or similar disability-related tasks.There is no license or registration process for service animals in the United States. A primary goal in revising the definition was to reduce abuse and fraud committed by people who falsely claimed that their cats, birds, ferrets, reptiles and other pets were service animals.While the ADA has narrowed the definition of service animals that are required to be permitted in places of public accommodation, other laws may still provide broader definitions in other areas. For instance, the Department of Transportation's regulations enacting the Air Carrier Access Act permit "permit dogs and other service animals" to accompany passengers on commercial airlines.  The Fair Housing Act also requires housing providers to permit service animals (including comfort and emotional support animals) without species restrictions in housing. Source: http://en.wikipedia.org/wiki/Service_animal