Mycoplasma and mycobacterium refer to two kinds of bacteria groups.  Mycoplasma strains can cause unusual or atypical pneumonia and respiratory diseases.  Mycoplasma differ from other bacteria types, including mycobacterium, in having no cell wall -- the normal part of a cell that divides a cell from the external environment.  This makes mycoplama strains resistant to many kinds of common antibiotics like penicillin, that act on bacteria cell walls.  Species, or types, of mycobacterium are responsible for dangerous diseases in humans including tuberculosis and leprosy.  Although mycobacterium have cell walls (unlike mycoplasma), their cell walls are unique, and also resist treatment by many common antibiotics like penicillin.