Akuma are the monstrous creatures that exist in the D.Gray-Man series. They are created by the Millennium Earl, the main antagonist of the series, and serve and take orders from him and the Clan Of Noah. They are dangerous monsters who have no other purpose but to attack and kill humans and the more humans they kill the more powerful they become.
An Akuma in the D.Gray-Man universe is created through tragedy: when someone loses a loved one, and they are in mourning, the Millennium Earl appears before them and grants their wish of bringing their loved one back by summoning the souls of those who died inside a skeleton machine, however the souls are in pain and suffering after this ordeal, and, under the Millennium Earl's orders, they attack and kill the loved one that asked for their resurrection and wear their body like a skin. The combination of soul trapped in the skeleton machine and wearing the body of a human makes an Akuma.
Akumas can also evolve by killing humans. And as of right now, they have 4 stages of evolution which they can achieve the more humans they kill.
It's the job of an Exorcist in this series to hunt down and destroy Akuma so that they may stop an Akuma from causing more carnage and also bring salvation to the soul inside an Akuma.
