The U.S. Department of Justice issued revised Americans with Disabilities Act (ADA) regulations that took effect on March 15, 2011. Under the revised ADA regulations, public and commercial facilities such as restaurants, hotels, retail stores, taxicabs, theaters, concert halls, and sports facilities must permit service animals to accompany people with disabilities in all areas where members of the public are allowed to go. Private businesses and government agencies must make reasonable modifications of policy and procedure to allow miniature horses trained to act as a service animal as well, when reasonable. A service animal is a working animal; not a pet. An example of a task a service animal can perform is providing a safety check or a room search for a person with Post Traumatic Stress Disorder (PTSD).
Under Title I of the ADA, the employment section, there is no definition of "service animal." A service animal in the workplace is just like any other reasonable accommodation in the workplace and may not necessarily be defined by the regulations as stated in Title II and III regulations.
For additional information, take a look at the following resources:
FAQ: What if a service animal barks or growls at other people, or otherwise acts out of control?
FAQ: How can I tell if an animal is really a service animal and not just a pet?
Service Animals, Small Business, and Other Public Accommodations