Postal is a 3D shooter with mainly isometric, but also some top-down levels featuring hand-painted backgrounds. Gameplay and interface are similar to first-person shooters of the time in most, but not on all counts:
Movement is always relative to the orientation of the player character ("The Postal Dude"). The player therefore must always be aware of the direction the character is facing, which can be difficult to some players on the isometric maps.
There are eight weapon slots, each with a fixed amount of maximum ammo. The default weapon is a weak machine gun with unlimited ammo. Although it serves no practical purpose, the player can conceal their weapons by pressing the tilde key.
Contrary to first-person shooters, however, the goal is not to stay alive and just reach the next level, but to kill a given percentage of the armed NPCs on the map. Only then the exit to the next level is activated. Even if the player is dead, they may still exit the level as long as the required number of hostiles have been killed (most likely a glitch).
There is no plot as such. The presence of a moving van on the first level suggests that the Postal Dude has been evicted from his home and is therefore "going postal", but no background story evolves during the game. However, the manual hints at a possible backstory (the Postal Dude believes the town's population has been infected by a madness plague), and in-between levels, players are given a chance to delve into the Postal Dude's psychosis with ominous diary entries containing phrases like "The earth is hungry. It's heart throbs and demands cleansing. The earth is also thirsty...". The game ends with the Postal Dude attempting to massacre an elementary school, but failing due to having a mental breakdown and ending up getting captured by the government and incarcerated in an asylum.