The zombie apocalypse usually starts by some kind of outbreak of a ‘zombie plague’ due to an infectious disease. Initial contacts with zombies are extremely dangerous and traumatic, causing shock, panic, disbelief and possibly denial, hampering survivors’ ability to deal with hostile encounters.
The zombies are usually bestial, motivated by a need to consume living humans or at least fulfill an imperative to re-transmit an infection by biting or clawing their victims. Those killed by or infected by zombies usually turn into zombies themselves. The plague spreads quickly, exponentially, and is unable to be controlled.
The authorities appear to be slow to understand the true nature of the threat, first responding with inaction or inappropriate tactics and thus giving it time to grow beyond hope of containment, leading to the panicked collapse of civilian society.
Often the specific cause for the reanimation of the dead is indeterminate or, if known, cannot be controlled by broad countermeasures with available technologies. In either case, there is no way to stop the dead from rising after the phenomenon has begun. The survivors barricade themselves in protective shelters and are forced to scavenge for food and supplies in a world reduced to a pre-industrial hostile wilderness.
The stories usually follow a single group of survivors, caught up in the sudden rush of the crisis. The narrative generally progresses from the onset of the zombie plague, then initial attempts to seek the aid of authorities, the failure of those authorities, through to the sudden catastrophic collapse of all large-scale organization and the characters’ subsequent attempts to survive on their own.
Such stories are often squarely focused on the way their characters react to such an extreme catastrophe, and how their personalities are changed by the stress, often acting on more primal motivations (fear, self-preservation) than they would display in normal life.