An artifact is a special individual of a certain type of item. That type is called its base item type. Artifacts have all the properties of their base items, plus special abilities.
For example, the Eye of the Aethiopica is an artifact amulet of ESP. It will have the same randomized appearance as an amulet of ESP. You can wear it for extrinsic telepathy or class-name it, just like any other amulet of ESP, because that is its base item. In addition to that, it has special artifact powers no other amulet of ESP has. For example, it will make your power regenerate faster.