A therapy dog is a dog that might be trained to provide affection, comfort and love to people in hospitals, retirement homes, nursing homes, schools, hospices, disaster areas, and are defined but not covered or protected under the Federal Housing Act or Americans with Disabilities act. They also do NOT have public access rights with exception to the specific places they are visiting and working. Typically the dog would be granted rights by individual facilities only.
Therapy dogs are usually not assistance or service dogs, but can be one or both with some organizations. Many organizations provide evaluation and registration for therapy dogs, sometimes with focus on a particular therapeutic practice such as reading to dogs.