The document discusses audio-visual aids and their classification. It defines audio-visual aids as sensory objects or images that stimulate learning. It then classifies audio-visual aids into two main categories: projected aids and non-projected aids. Projected aids include films, filmstrips, slides, opaque projection, and overhead projection. Non-projected aids include graphic aids like photographs, posters and charts, display boards, 3D models, objects and specimens. The document also discusses the principles, advantages and historical use of audio-visual aids in education.