From Diablo Wiki
A classification of new group of monsters in Diablo 3.
The Reaper classification is for monsters invading the mortal realm of Sanctuary that have been created or corrupted by Malthael. These creatures include Death Maidens, Punishers, Executioners, Ghostly Seraphs, Exorcist_(monster)s, War Beasts, as well as list of the undead like Revenants. Reapers differ from demons because they are created through tainted angelic power instead of demonic power.