Stainless Steels are broadly divided into four types:
There is a fifth specialist type Precipitation Hardening Steels.
Molecular Structure of Steel
The fundamental difference between them is their crystaline structure. Steel is an alloy of iron and carbon. At normal temeperatures the atomic structure is that of a cube with an atom of steel at each corner and a single atom in the centre of that cube.
This is known as "ferritic" and, is by the way, magnetic.
When heated to about 900o C the structure changes and each face has an atom at its centre. This is the austenitic structure and is non-magnetic.
When ordinary steel cools down gradually it reverts to a ferritic structure. If you cool it fast, it will adopt another structure with the carbon atoms being arranged in one direction. This is martensitic steel and in its "as quenched" condition is hard but brittle and generally requires further treatment before it can be used.
By adding elements such as nickel, manganese and copper which favour the Austenitic structure the steel will retain its austenitic structure at normal temperature.
Austenitic steels are generally non magnetic but cold working can induce a degree of magnetism. They have good formability and weldability, as well as excellent toughness, particularly at low or cryogenic temperatures. Austenitic grades also have a low yield stress and relatively high tensile strength.
While austenitic steels are more expensive than ferritic stainless steels, they are generally more durable and corrosion resistant than other types.