If you want to know what the government can do, specifically, you can look at Article I, Section 8 of the Constitution, which lists Congress' powers: it can levy taxes, declare war, and coin money, among other things. These are called the government's enumerated powers, also called expressed powers, or delegated powers.
The immediate problem faced by the framers of the Constitution was that it was impossible to list all the powers of government. First off, there's just too many; and second, they wanted to build a government that would last and stand the test of time. They knew that the world was going to change; remember, these were all very educated and enlightened men, raised in the 18th century, the Age of Reason. They knew they couldn't predict what the world would look like in even ten years' time, much less in a century.
