What is a protectorate?

[Amaz]

and how do you get one?

[Akbar]

It´s a thingy that appears most of the time on the diplomacy options....that nation gives you some money each turn and an alliance if they accept (bribes recommended). Quite beneficial.

[eatme]

Half the income of protectorate state should go to the treasury of Protector. Big minus that protectorate may get involved into war and drag Protector into it as well.