THERAPY DOGS are dogs that might be trained to provide affection and comfort to people in hospitals, retirement homes, nursing homes, schools, hospices, disaster areas, and to people with autism. Therapy dogs are usually not assistance or service dogs, but can be one or both with some organizations. (source)
The Disabled World website covers the subject very well.
Pet therapy, properly known as animal-assisted therapy (AAT) is a type of therapy that involves an animal with specific characteristics; becoming a fundamental part of a person’s treatment. (source)
Read first. How to get your dog involved in pet therapy.
Additional reading. Book, “Izzy and Lenore” by Jon Katz. Day-to-day ups/downs for handler and dogs who do pet therapy.
Additional reading. Precautions that therapy dog handlers should take to protect dogs and patients.
Love on a Leash: (three chapters in Missouri)
Pet Partners (formerly known as Delta Society)