Basically, it's another series touching upon relations to the soul--as such, you can expect my typical barrage: God is real, so are heaven and hell, yet neither is permanent; souls can reincarnate. Souls are destined for one or three places on death: immediate reincarnation, heaven, or hell. Enforcing this? Collectively known as Seraphim (the probable title of the webcomic), Angels (souls volunteering from heaven) and Reapers (souls serving a punishment in community service to avoid hell).
You'll note that vital detail: Yes, angels and reapers, beings of small-god-like power, were once human. That's because human souls are one of the most powerful forces in existence. Sometimes, they need help, or to be forced, to where they need to go. Since they sometimes fight back, angels and reapers work in pairs of two. (It also is a buddy system: if your partner goes rogue, you're right there to stop them.)
There's more to souls than that, of course. Souls, when reincarnating, stay basically the same person--in fact, any soul in heaven remembers all of its past selves. (Souls are genderless; as a result, angels and reapers can choose what type of human form to take.) However, it is possible to 'kill' a soul. Mainly, this is what happens to angels/reapers killed in the field by angels/reapers that have gone insane, though it can have other causes. A killed soul loses all past selves and their present self, being forcefully reincarnated as a new person.
Now, about angels: they volunteer, having the ability to travel between worlds at will. Their boss is Gabby, the currently-female archangel Gabriel(le). They're not supposed to interfer with humanity unless God orders them to give guidance. They use flaming swords of simmering light, wings to fly, and are garbed in white (when not in civilian clothing) with gold accents.
Luke (who often changes to Lucy) was the first angel, and he went insane. 40% of angels rebelled with him. After half the angel population was dead, his insanity was cured, but he insisted on atonement, becoming The Great Reaper, first of his kind. The surviving fallen angels became the first generation of reapers.
Fallen angels are indistinguishable from other reapers, except they keep their (now blackened, bloody) sword; reapers that are born as reapers are issued a scythe instead. Their current boss is Mike--yes, as in, former-archangel Michael. Think of him as like Oma from Stargate SG-1: He fell not because of insanity like his 'big brother', Luke, but rather, because he keeps breaking the same rule: he likes to interfere in human lives to make them better, which is a violation of allowing truly free will. So while he means well, he is breaking a rule, thus, reaper, not angel.
If you're wondering about reapers: they serve sentences, roaming the earth, stuck in mortal form. They don't age (souls are ageless), and don't require food or drink...but they do get hungry/thirsty. Meaning, they'll eat anyway. They go incorporeal to deal with souls, but are otherwise in a human body full-time. They can fly like an angel, but use a black tattered cloak/robe covering their entire body, hovering as their mode of flight.