Is The US a democracy?

Is the US a democracy?  What form of government does the Constitution set up?  Help give 3-4 examples to illustrate and explain that form? If this is not democracy, then give 3-4 examples of how the US or state governments can be argued to be democratic.  What are separation of powers and checks and balances and how do they fit into determining the form or government in the US?  Would the US be better served by a fusion of powers or more authoritarian leadership?  Why or why not (back with 2-3 pieces of evidence and/or examples)?