Settler (Fallout 4)

A settler is any person who decides to join a settlement. They can be found all over, especially near settlements.

Settlers in allied settlements
Speaking to a settler from one of your settlements will allow you to access their inventory and select their equipment, much like you would a companion. They can be armed with better weaponry than their pipe pistols and don't seem to consume ammunition(they do need to have ammunition in their inventory to be able to fire.) They only need 1 ammo of the type their gun uses and they will be able to fire at will and not consume any ammo (confirmed by multiple people here: www.gamefaqs.com/boards/164592-fallout-4/72809150 ) The exceptions are nukes and missiles; these will be consumed.

Settlers assigned as shopkeepers will have other dialogue options when spoken to similar to other merchants. Select "trade" from the dialog tree to access their inventory, speaking to a shopkeeper settler outside of their opening hours will go straight to their inventory as normal. Doctors seem to be an exception, a settler assigned to a doctor stand will have not have a trade dialogue option when spoken to during business hours. To access their inventory either wait for the shop to close or unassign and reassign them from their shop.

Appearances
Settlers appear only in Fallout 4.