NPCs in Elden Ring are computer controlled characters scattered throughout the world of Elden Ring. You can trade with or follow their quests for an (most likely) unique reward and most likely meet them in certain different spots throughout your journey. Some merchants might change the goods they provide after reaching progress in the story, so paying them a visit is always recommended. You can sort NPCs here by Merchants and Story Characters which most likely provide a questline to follow.
For further information about NPC questlines visit the Quests