Pure iron is soft, bends easily and rusts, so it is not much use for building things. Iron is therefore alloyed with carbon and often with other metals to make steels. The added atoms are a different size from iron atoms, so they stop the layers sliding and make the metal stronger and harder.
The amount of carbon changes the properties. Low carbon steel is softer and malleable, which means it can be pressed into shape, so it is used for car bodies. High carbon steel is very hard but brittle, so it is used for tools. Brittle means it snaps rather than bends.
Stainless steel is an alloy of iron with chromium and nickel. It is hard and does not rust, so it is used for cutlery, sinks and surgical instruments. Different alloy steels are made so that the metal has the right properties for its job. The chromium is what gives stainless steel its resistance to corrosion.