First off, Massage Therapist is the preferred term, not masseuse or masseur. The latter names have negative connotations. I’ll explain why in a minute.
Massage Therapy is the medical science of body work. Essentially, as my teacher has explained, massage therapists are ‘doctors of muscles.’ A massage therapist must know almost every muscle in the body; not only that, but they must know insertion and origin (attachment points/ the way the muscle pulls), the actions, the innervations, and other little tidbits that pop up along the way. A massage therapist must also know about skin, bones, and general body functions; a typical certification course will include extensive anatomy and physiology.