Horizon Forbidden West has several quest types:
Main Quests are the missions that carry you through the game's primary storyline, as you might expect. There are 17 Main Quests in Horizon Forbidden West.
Side Quests are optional objectives provided to you by other characters in the game's world, ancillary to the main story. There are 28 Side Quests in Horizon Forbidden West.
Errands are less pertinent missions Aloy can pick up over the course of her journey, often related to side activities such as Machine Strike or Melee Pits. There are 20 Errands in Horizon Forbidden West.
For a far more detailed breakdown of the quests in Horizon Forbidden West, see our guide: Horizon Forbidden West: All Quests.