Iron is an elemental metal, frequently used in medieval times for weapons, tools and armour. Its atomic symbol Fe derives from its Latin name ferrum. Due to its utility and the difficulty of mining and smelting and working it, iron was (and is) relatively expensive.
Steel is an alloy of iron and carbon, and quite uncommon in the medieval period.
Both steel and iron will oxidize, causing rust, which weakens the metal. For this reason, period armour was often painted or left covered in forgeblack, rather than being polished. If armour was polished, it needed frequent scouring and oiling to maintain it.