Stainless steel is made by adding chromium and nickel to steel, making it highly anti-corrosive. The most preferred type will be stamped "18/10" (the ratio of chromium to nickel added) though other types of stainless steel are also used with very good results. This material can be found in many of the best pots and pans because it is durable and attractive.
Stainless steel (particularly "18/10") is also prized as an interior cooking surface because it does not react with acidic or alkaline foods and won't pit or scratch easily. This quality is an important one, as it promises a certain purity to whatever gets cooked in it; it will not discolour foods or impart metallic flavours no matter what they are. Additionally, stainless steel is dishwasher, oven and broiler safe.