Great people can be settled in a city, where they provide a steady bonus of some kind to that city, or they can be expended to get an immediate and powerful bonus. There are six different types of great people; each type provides a different benefit to your civilization.
You cannot settle two great people of the same type in the same city.
Great People Types
Great Artists/Thinkers
Special Ability: Expend a great artist to convert a nearby city to your civilization! (The city must be visible and cannot be hidden by "fog of war.")
Settlement Ability: Settle a great artist in a city to increase that city's cultural output by 50%.

Great Builder

Special Ability: Expend a great builder to complete any unit, building or wonder under construction in that city.
Settlement Ability: Production costs of all future buildings in the city are cut in half.

Great Explorer/Industrialist

Special Ability: Expend a great explorer to gain an immediate large chunk of gold in your treasury.
Settlement Ability: Receive a permanent boost to that city's gold output.

Great General

Special Ability: Great generals appear after one of your units has won an exceptional victory on the battlefield. The great general immediately "attaches" to the successful unit; it provides a 50% attack and defense bonus to all units in that space.

Great Humanitarian

Special Ability: Expend a great humanitarian to increase the population of every city in your civ by 1.
Settlement Ability: Provides a 50% boost to the city's population growth rate.

Great Leader

Special Ability: Expend a great leader to immediately give all of your units the veteran upgrade.
Settlement Ability: Units constructed in a city with a great leader settled in it are automatically veterans.

Great Scientist

Special Ability: Expend a great scientist to immediately complete research on the tech you're currently studying. Settlement Ability: Increase the city's research output by 50%.
 
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