Canada is a constitutional monarchy. The Queen, represented by the Governor General, is the head of state and the Prime Minister is the head of Government. The U.S. is a republic. The head of state and head of the Government are the same person: the President.
The Constitution begins with the Preamble, which states the basic mission and goal of the document, and then goes on to describe the different aspects of the government in varies articles. And since the Founding Fathers wanted the Constitution to be able to "adapt" to new times, they allowed for amendments to be made to it.
