An akuma is a soul which has been brought back to life through sadness and despair with the powers of Millenium Earl. Their mission is to terrorize the earth and all who dwell within it. The Black Order is a group of Exorcists that have sworn to cleanse the souls of the akuma, and collect the substance known as "innocence". This powerful substance can be used as an anti-akuma weapon, and ultimately can help destroy the evil Millenium Earl. Allen Walker is a young Exorcist who has the ability to tell who is human and who is an akuma with the help of his cursed eye. Together with the Black Order, Allen must find all 109 pieces of innocence to protect the world and defeat Earl.
Both of the anime's main character can see demons, unlike the other characters in the series. There is a lot of action, comedy, and adventures in both anime. So if you like one you'd like the other.