The first citizens of each city usually work the land, each toiling to yield up the resources of one terrain tile. However, there may be other specialist roles citizens can assume; in fact, taking another role is the only way they can stop working. A city may outgrow the land available for it to work, in which case some citizens must become specialists.
All specialists enjoy enough privilege to remain perpetually satisfying -- they do not contribute to unhappiness or to celebration. See the section on Happiness for more details.
Each entertainer produces two luxury points for their city per turn, increasing to three once any player has built Shakespeare's Theater or Internet. See the section on Happiness for the effects of luxury points.
Each scientist adds two points to your empire's research output per turn (except of Anarchy or Fundamental government, which yield only one point), increasing to three once any player has built Darwin's Voyage or Large Hadron Collider.
Each tax collector produces two extra gold for your treasury per turn (except of Anarchy, which yield only one point), increasing to three once any player has built A.Smith's Trading Co.