A stainless steel is a material that resists corrosion and oxidation. This combination is what prevents rusting and unsightly blotches from developing. A thin oxide film forms on stainless steel when exposed to oxygen and moisture. Essentially, it is self-healing.
In spite of its name and resistance to staining, stainless isn't impervious to stains. Pitting and corrosion occur because the protective film is worn away over time.
Regularly cleaning the stainless steel surface and ensuring it has adequate oxygen are necessary for its maintenance.