A deficit occurs when spending or outgoing money is greater than revenue or incoming money. In the case of the U.S. government, the deficit for 2013 is projected to be $759 Billion Dollars. That means we are spending that much more than we are bringing in putting us further into debt.
Rough estimates for 2013 would be something like this:
Spending (Outgoing Money) = 3.7 Trillion
Revenue (Incoming money) = 2.941 Trillion
Deficit (New debt) = 759 Billion