An artifact, along with the closely-related relic, is a superpowered magic item. So superpowered, in fact, that it is certainly more powerful than your PC. The artifact is a creation of some ancient god, or hero, or villain; the relic is that god / hero / villain, at least part of him.
The model for the artifact is from Tolkien, who featured several magical items that you really should get off your hand (literally) as soon as you can, and not use in the meantime. An artifact should only feature as the main plot of a high-level adventure unless you're just taking the piss.
In D&D E. Gary Gygax introduced the concept(s) in the first Advanced Dungeons & Dragons Dungeon Masters Guide. That Wand of Orcus was an artifact mooted here, the destruction of which features in at least two published adventures. The Hand and Eye of Vecna are relics you can graft onto your own person - if you are a complete fucking retard, but it's all good because those items will help do the thinking for you from now to the end of your (un)natural life.
In BXCMI they feature in the Master's Set, so levels 26 up; the artifacts here all come from our own good green Earth, so if you're playing in, oh, Mystara they're to be used as a template only. Second Edition AD&D had a whole Zeb Cook book devoted to various artifacts (and relics).
For WotC 3e as TSR 2e, the first published material didn't post 'em. In the OGL environment White Wolf, as so often elsewhere, beat WotC to the punch - here, in Relics & Rituals (albeit with a Scarred Lands focus). WotC caught up in the Epic Level Handbook.