They are various people from all walks of life and can be generally sorted into four types:
These are people who have discovered they have an artifact and use it for self-serving and evil purposes. They are usually defeated by the agents, and sometimes by the artifact itself, which can turn on the user or be employed against them.
People who have an artifact but use it for selfless and good acts, though the nature of the Artifact can still draw the attention of the agents. They will generally give up an artifact of their own free will, or must be freed from the power of said artifact if it grows out of control.
People who have an artifact but don't recognize or realize its nature and power. These artifacts can be dormant, or be active and work for the user without them being aware of it, such as the Christmas Truce Submarine Ornament endeavoring to give Kallie her father back, or Walter Winchell's Tie Clip and Cuff Links being used by a lawyer.
People who were present at an extraordinary or important moment with an object that becomes infused with the energy of the moment that turns it into an artifact. Mrs Frederick explained to Claudia that the Warehouse leaves creators alone until their artifacts become a danger to the world at large. It is rather implied that a creator will often if not always employ the artifact instinctively and judiciously - it is later bearers who tend to seek unconsequenced gain.