Iron is an element, with atomic number 26. That means every iron atom has 26 protons, and how many neutrons it has depends on its isotope. Solid iron makes crystals with a regular lattice pattern, but iron crystals can have little defects in them -- missing atoms, extra atoms, places where the regular lattice patterns don't match up quite right. Steel is made by adding a small amount (usually less than 2%) of carbon. Adding carbon helps prevent defects in the iron crystals from sliding around, and makes the iron stronger.
Steel may have other ingredients added, to improve its properties: tensile strength, hardness, resistance to corrosion, elasticity, and other properties. Common ingredients are nickel, manganese, chromium, molybdenum, and vanadium. Stainless steel contains a lot of chromium (18%) and also quite a lot of nickel (8%), and is quite expensive.
(published on 10/22/2007)