Proteases can be classified into four main types - serine, cysteine, aspartic, and metallo proteases. Serine proteases contain a catalytic serine residue and include subtilisins. Cysteine proteases contain a catalytic cysteine-histidine dyad and include papain. Metalloproteases require a divalent metal ion like zinc and include thermolysin. The document discusses the classification, sources, and applications of various protease enzymes.