There are many NPCs in the world.
Quest NPCs
A very small amount of NPCs will give you quests, most of which require you to clear out a dungeon, but some will ask for you to retrieve an item. These quests are worth it, some giving you large amounts of gold, some giving the ability to learn new skill-sets (Barbarian, Monk, ect.). You can tell that they have a quest or quest followup by the exclamation point over their head.
Normal NPCs
These NPCs are the most plentiful, always walking around or standing in one place, repeating the same things if you speak to them. They usually have no purpose, unless you enter Crazy Mode.
Enemy NPCs
will attack you on sight