Artifacts of Power are often lumped into one category; "A powerful item that is very valuable", but they can have varied origins, even if the end result does end up in that single category.
The most common artifact of power, these days, are Demon Forged Artifacts. Demons LOVE making weapons, and stockpiling them, and eventually losing track of them. Armor, swords, shields, wands, horrific contraptions of unknown purpose, demons make them all. These tend to be very powerful, and very straightforward. If you have a demon made armor, chances are that it will help you kill things more easily. If you have a demon made ring, chances are it will help you kill things more easily.
The other major artifact of power are Pre-Fall "treasures". These were made in huge amounts and vary considerably, but the ones that have survived to this point in time tend to be very powerful and useful. These can induce flight, limited invulnerability, enhanced speed, and so on. While not as directly deadly, these are still invaluable.
The two rarest categories of Artifacts of Power are Pre-(H)istory weapons that still work (A HUGE rarity), and DIVINE artifacts. Divine artifacts are (supposedly) artifacts made by God herself. Of immense power and indestructible, these items have a vast range. A Divine sword could cleave a mountain. A divine spoon could never spill a drop of milk. Divine artifacts are almost immediately claimed by angels if found, and the finder will be compensated for handsomely.
(But like... if you can keep one for yourself... do so...)
-Thoughts of Monet Runeblade, Adventurer