The Balance is an abstract, semi-sentient energy that transcends throughout almost all realms and realities. It is comprised of a perfect blend of both the respective energies of Chaos and Order. One who utilizes Balance, called a Balance-Keeper (or Master of Balance), has enough power to alter reality, yet is slave to the Balance itself.
The Balance itself is a network of energy, threading its way across universes like cosmic string. The Balance can both affect and be affected by happenings within universes, and if it were to disappear altogether, reality can be thrown into Imbalance- the inverse of Balance. While The Balance uses Chaos and Order to promote creation, the Imbalance uses Chaos and Order to promote destruction. Equilibrium is the abstract force that holds the Balance and Imbalance at bay, yet is a force that no creature, mortal or immortal, can tap into.
The lines between the roles of Balance and Imbalance are blurred, however, and each can infringe upon the other's role at times.