The source is the being that released the God Wave that empowered DC's gods and super humans. Its realm is the source wall, which is at the edge of the DC multiverse, which traps all those who attempt to pass beyond it. DC Comics has kept the Source out of the main continuum of its story telling; therefore, not much is known concretely about its many aspects.
Death of New Gods (version)
The Source was attacked by three elder gods of the third world and was split in two, one being The Source and is the light side, the other is the Anti Life Entity. After that, The Source plotted to remake the forth world for it though it to be flawed. Its plan was to use the Infinity man to kill all the New Gods, and merge with The Anti Life Entity, then using the soul fire equation to remake everything. Darkseid eventually discovered its plan, and was able to steal the equation and battle evenly with the then merged Source, finally trapping it. Superman went on to find where Darkseid had hidden the souls of the New Gods, and released them so the Source broke free and defeated Darkseid, then declaring it time on this plane of existence was over.
Powers and Abilities
The Source originally is fully omnipresent, omniscient, and nigh omnipotent. Considered by some to be the 'Buddha' of DC, because it represents a shared cosmic consciousness in the DC Comics multiverse. It's power is far beyond even beings such as Lucifer Morningstar and Michael Demiurgos. Once The Lightbringer entered its realm the Source Wall without even acknowledging The Source, but The Source's omniscience made it not interfere as it know the consequences.
Death of New Gods
The Death of New Gods version is weaker. Also, in that series the Anti Life is not an equation, but an entity, the dark side of the Source. It has cosmic awareness, and unlimited power capable of destroying entire universes with ease.
While investigating the death of the New Gods, Metron encountered the Anti Life Entity, and he cried because he felt the Entity's infinite power; "I can feel the limitless power that is the entity. I now see what a God truly is. I've never felt so insignificant. Next to this deity, I am nothing". Darkseid described it as; "Call it death. Call it Anti-Life. It is beyond imagining. It is the complete unknown. It is fear."