Government types:

-Anarchy:transitory government:

You must get 5 turns of anarchy if you want to change covernment type.Many disadvantages.

-Tyranny:This is the best government if you like to make a lot of wars.Your military strength is greatly increased, but your tech as well as your Per capita income(`=Taxes) are quite decreased.

-Dictatorship:Looks like tyranny, but is more moderated:your military are less strong, but your technology and your pci are increased.This is also a war type government.

-Communism:This one is very special:you lose a lot of money, but productions are greatly increased.In generally, you switch to this government to prepare yourself to war by building more weapons and food.
Then you switch to tyranny or dictatorship and go to war.

-Republic:This is the perfect non-war government: Your army is quite weak, as you earn more money and your technology is more efficient.I prefer anyway the democracy.

-Democracy:The only disadvantage of this government type is the spy efficiency loss.You earn back this disadvantage by increasing your technology efficiency, and your PCI.this is a quite good government.

-Monarchy:No disadvantages, No advantages