Pitting describes the effect, and rusting the cause. Generally they're talking about the same issue, and the difference is the degree or depth of rust.

Others may also be referring to pitting from other causes, such as from chemical etching, or some people may intend a precise meaning, ie. pitting to mean deep pockmarks or dots (acne on steel) vs a broad area of rust, of any depth.

You also see people refer to pitting when describing plated steel, where you'll see rust dots (pits) because of the porosity of the plating. This type of pitting was classic on chrome auto bumpers for decades. Either way, it's all rust.

In any case, you can remove rust, but neither rust nor pitting are reversible. Once the steel is gone, it's gone for good.