There are two 'common' grades of stainless steel. These are A2 and A4. A2 is 'standard' stainless steel and A4 is classed as 'marine grade' as it can withstand salty atmospheres/environments better than A2.
Stainless steel is made by mixing the steel with chromium and, obviously, A4 will have a higher chromium content than A2.
A good way of telling if something is stainless steel or not is to hold a magnet to it (assuming it is actually steel). A2 will attract a magnet but the attraction will be weak and virtually unnoticeable while A4 will not attract a magnet unless the magnet is very powerful. So, if you have an item which claims to be stainless steel such as a kitchen knife etc and it attracts a magnet, then the claim is at best a little dubious.
Stainless steel does actually rust. It just takes a lot longer to do so than 'ordinary' steel.
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