Steel-type is one of 18 type classifications that is assigned to type-moves and Pokemon who's physiology are metallic or associated with it in some way. These types are most often dual-typed with others, with pure Steel-type populations being quite small by comparison. As a result, Steel-typed Pokemon can be found in just about any location. However, many Steel-types do appear to favor terrestrial locations, like mountains, cliff-sides, and other rocky locales.
Defensively, Steel-typed Pokemon are resistant to a large portion of other types, to include Grass-, Bug-, Fairy-, Dragon-, Flying-, Ice-, Rock-, Normal-, Psychic, and other Steel-type moves; though weak against Fire-, Ground-, and Fighting-typed attacks. Offensively, Steel-type moves are effective against Ice-, Fairy-, and Rock-typed Pokemon, while lacking in effectiveness against Electric-, Water-, Fire-, and other Steel-typed Pokemon.
Steel-types further enjoy an immunity to Poison-typed moves and effects, unless the user of those moves has the Corrosive ability. They are also immune to sandstorm weather effects.