Founded by the Guardians of the Universe, The Green Lantern Corps is an intergalactic "Police" department consisting of 7200 members (2 per sector of the universe).
The Honor Guard, troubleshooters and Special Ops group of the GLC, and are not restricted to a single sector of space. Guy Garnder was the first inducted into this group.
The Corpse, a black Ops division of the GLC that do NOT use power rings or the recognizable Green energy but, instead, theirs is Purple, and they prefer black suits with no GL distinction on them.
The Alpha Lanterns - Much like an IA (Internal Affairs) division of the GL, they have had their power battery surgically implanted, their Power Ring fused to them, and have been changed into cyborgs and had their minds mainlined directly into the Book of Oa.