Stainless Steels constitute a large family of iron-based engineering alloys containing a minimum of 12% Chromium. When you see 18/8, 18/10, 25/20 etc. the first is always Chromium and the second is always Nickel.There are several groups of stainlesses, known by their crystal packing or microstructural arrangement.
These are: (generally)
Austenitic - 300 series = 17 to 20+ Cr, 8 to 20+ Ni = non-magnetic.
Ferritic - usually less Cr and no Ni = magnetic.
Martensitic - 400 series = <18 Cr <5% Ni + Carbon for hardenabilty - magnetic.
Also duplexes and superduplexes = 25%Cr/20%Ni = non-mag, very corrosion resistant.
And also others - have mixed phases for special uses.
Chromium provides oxide film on surface and hence corrosion resistance.
Nickel provides strength mainly.
(not my own writing, copied from somewhere else)