Factions
Also said as Guilds, Reputation groups, Sides
Groups with their own goals, beliefs and grudges, often against each other. Helping one usually costs you with another.
Factions turn a backdrop into politics, and they make your choices cost something. They are also an efficient way to write a world: one good disagreement can drive dozens of quests.
Say it
“Give the river towns three factions: the ferrymen, the bridge builders and the toll office. Each favour moves your standing with all three, and no one can be friends with all of them by the end.”
Where you’ve seen it
Fallout: New Vegas sets the NCR, Caesar's Legion and Mr. House against one another over one city, and each faction keeps its own opinion of you. Yes Man offers a fourth way, which is to push all of them out.
Not to be confused with morality system
A morality meter has one judge. Factions have many, and an act that one group loves is the reason another group shoots you.