An artifact type is any subtype used for the further classification of artifact cards.
Other then creature types artifact types don't have to appear on all artifacts. Instead they are only introduced for mechanical purposes.
From the glossary of the Comprehensive Rules (July 3, 2020—Core Set 2021)
- Artifact Type
- A subtype that’s correlated to the artifact card type. See rule 301, “Artifacts.” See rule 205.3g for the list of artifact types.
From the Comprehensive Rules (July 3, 2020—Core Set 2021)
- 205.3g Artifacts have their own unique set of subtypes; these subtypes are called artifact types. The artifact types are Clue, Contraption, Equipment (see rule 301.5), Food, Fortification (see rule 301.6), Gold, Treasure, and Vehicle (see rule 301.7).
Clue is a subtype unique to tokens created by Investigate.
Contraption was only mentioned on Steamflogger Boss until Unstable featured cards with this subtype. They are played from a separate Contraption deck.
Equipment cards enter the battlefield just like any other artifact, but may be attached to creatures using their Equip ability. Unlike Auras, however, if an Equipment is attached to a creature and the creature leaves the battlefield, the Equipment remains on the battlefield.
Food can be sacrificed for life.
Fortifications act just like Equipment above, except that fortifications attach to lands using Fortify.
Gold artifacts (Sacrifice this artifact: Add one mana of any color) were introduced in Theros Beyond Death.
Key appears on Corrupted Key, a test card in the Mystery Booster set. It has no rules meaning or function by itself, but other cards and abilities may refer to it.
Treasure is a subtype unique to tokens created by some cards from Ixalan block and later sets. Similar to Gold, except that they have to be tapped.
Vehicle is an artifact which can temporary become an artifact creature by assigning a crew.