Treaty, Constitution, Charter, Contract
Aren't these really just words used to define a quid quo pro of some sort? They basicly define the same sort of thing: An agreement amoungst various parties to both give something up and recieve something in return. Of course some agreements give up something for others or on behalf of others...But I digress.
I know that is an oversimplification...But keep in mind that most people are not lawyers (thank GOD!)