Civilizations are playable factions, each of which represents a historical nation, empire or cultural group. Each civilization in Civilization VI has a unique ability and two unique components: a unit and a piece of infrastructure, which may be a building, a district, or a tile improvement.
Civilization VI makes a distinction between a civilization and its leader, not unlike Civilization IV - each civilization can now have multiple leaders, and each leader comes with his or her own unique ability and pursues a specific agenda throughout the game (when controlled by the AI). Some leader abilities also provide an additional unique unit. So, the civilization's unique unit, infrastructure and ability remain unchanged, while the leader ability and agenda are affected by the leader chosen. Thus the same civilization may behave quite differently throughout the game, depending on which leader leads it.
VanillaCivilization VI included 18 different civilizations at the launch, with the Aztecs (19th) as a pre-order bonus, to be released for everyone 90 days after the launch. For now, there is only one civilization with multiple leaders: Greece, which may be led by either Pericles or Gorgo.
All cities you found or conquer start with a Trading Post. If in Trade Route range of your Capital, they also start with a road to it. Trade Routes earn +1 Gold for passing through Trading Posts in your own cities.
May build Freets and Armadas earlier than usual (Mercantilism). Trade Routes between multiple continents receive bonus Gold from routes to other civilizations, and bonus Food and Production for routes between your own cities.