Since there is no true hell there is also no true heaven. This plane is, however, the closest that reality comes to such a place as heaven. When a soul chooses to let go of the anger, hate, and hurt of the world it passes here. The processes of letting go of things of the material plane is a difficult one, here a soul is safe and allowed to explore its life and the lives of others to gain meaning and enlightenment. Once the soul lets go of all the negative aspects of the world it tends to take on a cause of enlightenment, seeking understanding in some deeper way than mortals are capable of.
I would really, really, like some help fleshing this out into something more detailed. While it is mostly for Embers of Soteria the planes of existence are cross-setting.