The use of qualified service animals is protected by the Americans with Disabilities Act (ADA).
Any breed of dog can be considered a service animal under the ADA if it has been individually trained to perform specific tasks for a person with a disability. An additional provision requires that establishments allow the use of a miniature horse in place of a dog if it has been trained and if it's reasonable to accommodate.
Service animals have a specific and legally-protected purpose and are therefore different than emotional support animals, comfort animals, or therapy animals, although they certainly may provide emotional support, comfort, and therapy in addition to their trained tasks.
Animals have been trained to help humans for thousands of years. In this context today, people often think about dogs that guide people who are blind, sometimes calling them Seeing Eye dogs. The Seeing Eye is actually the name of an organization incorporated in 1929, born out of the need to improve mobility for blinded veterans of World War I.