It's easy enough to pick up a weapon and start blowing away enemies, but some games allow for the option of not killing anyone in order to progress, instead providing alternate ways around taking the life of somebody. This can include knocking them out, disarming them, drugging them, or simply sneaking past them. Often a self-imposed challenge taken up by the player, and occasionally rewarded with an achievement/trophy.
Note: there are a few cases of pacifist runs in games that still force the player to make a kill (such as Deus Ex: Human Revolution, Alpha Protocol, and Mirror's Edge). These unavoidable kills are typically story related, and often boss battles, but the game can still be considered for Pacifist Runs due to the nature of the way the game is played the other 99% of the time.