NOTE: SOME OF THIS IS NOT PUBLIC INFORMATION ICly. IF IN DOUBT, ASK.
The Custodians are the beings who created the Multiverse. While it would take up too much space to explain adequately each and every being that is a part of them, this news file summarizes it all.
The Custodians are a group of beings from a variety of universes who were united by the as-yet unnamed Chief Custodian, who has some special agenda that requires the creation of a Multiverse. Prior to the current Multiverse in-game, they have created at least one previous Multiverse, which collapsed into an "anti-reality" being that came into the current incarnation of the Multiverse, and was called 'Oblivion'. The Custodians themselves are of no one specific race, and most of them are from universes entirely unrelated to one another's.
Every Custodian has TP-Level reality-bending skills or exceptional intellect that makes them useful to the creation of something like the Multiverse. The abilities they have can be the ability to knit together land masses, delay or cancel out the effects of time on groups of beings, the creation of atmosphere, the large-scale transportation of matter (planets), enough understanding of the rules of any given universe to make things "fit" when the puzzle comes together.
Custodians are, by and large, not human. There are humans amongst them, but for the most part they're completely alien beings and humans are in the minority. While they have the ability to cause massive, worldwide changes, and some people regard them as phenomenally powerful on a cosmic scale, most of them are not even capable of defending themselves when directly confronted. That said, there are a small number of beings you might call "True Demi-Gods" amongst them, and their Chief is in fact what one would call a "True God", albeit still being subordinate to "God and Satan", as outlined in News Spirituality.
The Custodians are split into two groups. One is essentially "good", consisting of relatively low-power individuals who were forced into assisting by the Chief Custodian and his subordinates, and the other is, bluntly put, pure evil and out for their own gain. Most Custodians became Custodians by either being offered a "position" in a grand project by The Chief or his subordinates (most likely the latter), or being outright forced into it on threat of death or worse.