Grim Reapers are entities that appear as skeletons in black hooded robes, often seen carrying scythes. Their job is it to collect the souls of the deceased and bring them to the afterlife. After that, the deceased person can become a Reaper him- or herself, by applying for the job, which is what Natalie Durand did. In the beginning she didn't yet look skeletal at all, but that changed over time , at least for the eyes of mortals.
The base of operation for their organization Reaper & Soul is in the Land of the Dead , but at least one Reaper, Emissary Yama, is "living" in Hell. The title might imply that s/he could be some kind of ambassador.
Another Reaper once had to haggle with Erin Winters/Mordawwa over the soul of Eustace Boyce. They eventually came to an agreement when Erin offered to swap his soul for hers, even though the Reaper did not exactly look forward to doing the paperwork needed for this.
Learn more about the Grim Reaper from real world folklore here.