A regular steel is used on a knife that is in good condition but could use just a bit extra sharpness. It doesn’t actually remove any material from the edge but it realigns the microscopic metal parts at the edge. Therefore a regular steel can be used frequently without causing much wear.
Polished / Smooth Steel
A polished steel is much like a regular steel, but is used to achieve a much finer edge. When used on a knife that is in good condition you can bring up a razor sharp edge.
In contradiction to a regular steel, a ceramic steel does remove some material from the edge of the knife. Therefore it can be used on knives that can not be sharpened using a regular sharpening steel anymore. A ceramic steel may break if dropped on the floor.
Diamond steels give the quickest result, provided that they are used the right way. They will give your knife an edge in just a few strokes, but remember when using a diamond coated steel you are removing material. This method will eventually remove enough material to make the edge too thick to sharpen with a steel, requiring the knife to be reground.