From StrategyWiki, the free strategy guide and walkthrough wiki

Each civilization will possess one of several Government type that provides global modifiers across all of their units and/or cities. The default starting government is a Despotism, although several civilizations begin with another more advanced government as a result of a civilization bonus. Governments may be changed at any time so long as the required technology has been researched. However, changing governments will cause a turn of Anarchy, during which all outputs of all cities are reduced to zero. India and Japan are exempt from the anarchy penalty, although Japan only obtains anarchy immunity past the Industrial Age.

Government	Effects	Required Tech
Despotism	Can launch nuclear weapons without a culture loss	None
Republic	Settlers require only 1 population (instead of 2)	Code of Laws (3)
Monarchy	Double culture bonus from Palaces	Monarchy (4)
Democracy	+50% Science, +50% Gold, but may not declare war or decline a peace treaty	Democracy (4)
Fundamentalist	+1 Attack to all ground units, but Libraries and Universities no longer provide bonuses	Religion (5)
Communism	+50% Production, but Temples and Cathedrals no longer provide bonuses	Communism (8)