There are three levels to Facebook's campaign structure: campaign, advert set and advert.
Campaign: A campaign contains one or more advert sets and adverts. You'll choose one advertising objective for each of your campaigns.
Advert set: Advert sets contain one or more adverts. You'll define your targeting, budget, schedule, bidding and placement at the advert set level.
Advert: The creative you use makes up an advert.