...are service dogs individually trained to perform tasks which mitigate the psychiatric disabilities of their disabled partners.

There has been some confusion and some heated debate about psychiatric service dogs (PSDs). First let's clarify the difference between a therapy dog and a psychiatric service dog. A therapy dog is an individual's pet which has been trained, tested, registered and insured to work in hospital, nursing home, school, and other institutional settings. The therapy dog and his partner visit to cheer patients, to educate the community, to counter grief and stress, and generally be good canine ambassadors within the community. The canine partner of Delta Society Pet Partners is an example of a therapy dog. Most therapy dog partners are volunteers, but some states recognize professional therapy dogs partnered with therapists and other mental health professionals. Therapy dogs are not service dogs.

Under U.S. law, persons with therapy dogs are NOT granted the right to enter businesses with their dogs which do not permit pets. They do not get to fly in the cabins of aircraft because they are therapy dogs, nor do they get to live in "no pets" housing because they are therapy dogs.

Dogs used for emotional support, that are not task-trained, are called emotional support animals. They are not service dogs.

"The Department is proposing new regulatory text in § 35.104 to formalize its position on emotional support or comfort animals, which is that ‘‘[a]nimals whose sole function is to provide emotional support, comfort, therapy, companionship, therapeutic benefits, or promote emotional wellbeing are not service animals.’’ The Department wishes to underscore that the exclusion of emotional support animals from ADA coverage does not mean that persons with psychiatric, cognitive, or mental disabilities cannot use service animals. The Department proposes specific regulatory text in § 35.104 to make this clear: ‘‘[t]he term service animal includes individually trained animals that do work or perform tasks for the benefit of individuals with disabilities, including psychiatric, cognitive, and mental disabilities.’’ This language simply clarifies the Department’s longstanding position."
-- http://www.ada.gov/NPRM2008/t2NPRM_federalreg.pdf