A therapy dog is not a service dog. It is one that is tested, registered, and insured to go with its owner to visit facilities like hospitals and nursing homes to cheer up patients. Note that like other pets, therapy dogs are only permitted where they are invited. You still have to have permission from the hospital to visit, so it pays to check with the facilities you would like to visit and see what they require. Some will only accept therapy dogs registered with a specific organization.
In the UK it's Pets As Therapy (PAT)
Each has it's own test and registration requirements, and each provides liability insurance for its members.
Therapy dog registration is based primarily on temperament. The obedience requirements are minor: walk on a loose leash, sit, down, and come when called are about it. For most well-behaved pets, an eight week course would be sufficient to prepare for a therapy dog test.