The Steel type is one of the eighteen official types from the Pokémon franchise. It marked its debut in the Generation 2 games and was specifically created to tone down the Psychic type's prowess and raise the power of the Fighting type in order to make the Pokémon games more fair. Steel is an extremely sturdy type, actually being the most defensive type of the Pokémon series, and thus being the best choice for a defensive team.
While an average Steel Pokémon lacks Speed, its physical Defense is outstanding, making a Steel type an outstanding physical tank in battle. Out of all the eighteen types, they are able to resist 10 (including the newly introduced Fairy type), and are also immune to Poison-type attacks and the Poison status condition. They are, however, weak to Fire, Fighting and Ground, three commonly used types for several moves.
Offensively, the Steel type can only deal super-effective damage to three types: Fairy, Ice and Rock, whilst resisted by Electric, Water, Fire, and Steel itself.
Due to the varying weaknesses and fewer resistances of other types, most Steel Pokémon with a second type don't have as many resistances as a pure Steel type. However, certain ones do have only a few weaknesses in return, or in the case of Pokémon like Scizor and Ferrothorn, only one.
As of Generation 6, there are about 42 Steel-type Pokémon, making it the 5th rarest type. With only about 5 pure Steel-type Pokémon, the Steel type is much more common as a dual type. With 21 primary Steel types and 22 secondary Steel types, the Steel type is about as common being primary as secondary.
Steel-type Pokémon mostly consist of steel, iron, silver, copper, bronze, or any other kind of metal (or element pertaining to metal). As such, a handful of Steel Pokémon are part of the Mineral Egg Group. Some Pokémon, like Metagross or Magnezone, are magnetic as well. Steel-type Pokémon can also be armored, covered and/or plated with metal, like Skarmory or Bisharp. Most Pokémon like that aren't part of the Mineral Egg Group.