A point of divergence (POD) is the point in time when a decision or random event changes the universe as we know it, i.e. "Our Time Line" (OTL) into a parallel universe, aka an "Alternate Time Line" (ATL).
Based on the science behind "quantum mechanics" (a sub-atomic phenomenon), it has been postulated that an infinite number of parellel universes (or a "multiverse") exists wherever possiblilities diverge. The most obvious source of possibilities is that of human free will. A person faces a decision and chooses one thing over another. In an alternate reality, he choses differently, changing the future in the process.
The driving force of alternate histories is the "butterfly effect " as things change little by little until big differences become evident. In writing of alternate histories, an author assumes that major events and/or decisions are different, setting in place a chain of events that alter history. The more major the event, the more different the new reality will be. However, alternate realities would be created by every different decision made. Most of these realities would be indistinguishable to a historian who follows world events across the "multiverse."
Major battles -- especially ones that were close -- are favorite POD's. Often these battles determine the course of wars, and thus nations. Favorites among alternate historians are different outcomes for the American Civil War and the Second World War (see Spark!).
On the wiki "althistory.wiki.com" a popular time line with multiple authors was the community effort 1983: Doomsday . In this case a Soviet officer's cool head quite possibly saved the world from "Mutually Assured Destruction" in OTL. In the ATL, though, another officer is on duty. The result is the near destruction of the Northern Hemisphere in 1983.