Honing steels! Ceramic vs diamond vs regular hardened steel
Wow, this can be pages long if we want to. Ok, quick answer.
Regular hardened steels are usually grooved. It is meant to realign a bend knife edge. I believe a polished smooth steel is better than a regular grooved steel.
Ceramic and diamond rods also realign the edge, but also sharpening the knife edge in a limited degree. They are better. Usually, ceramic rods are finer than diamond rods.