Quenching or hardening is heating up of steel article to redness (1123K) and then cooling it suddenly by plunging it in water or oil to make steel hard and brittle Annealing is heating of steel well below red hot and then cooling it suddenly to make it soft.
Tempering is heating the quenched steel at around 550K and then cooling it slowly to make it quite hard but less brittle Nitriding is heating of steel in atmosphere of nitrogen to make its surface hard