NPC-driven factions are official, like the Vault City faction or the Enclave faction. Joining one of them is useful, because you will be able to meet instructors for professions, and to make some private quests. But you won't be able to be in a player-driven faction until you resign from your current one. You can join and leave a faction freely, but (unlike player-driven factions) it will affect your reputation, so be careful. See the factions category for more details. Joining or resigning from a NPC-driven faction will change drastically your reputation.
This category has the following 3 subcategories, out of 3 total.