What is the difference between consumption and government purchases?

What is the difference between consumption and government purchases?
In macroeconomics, GDP is defined as: Consumption+Investments+Government Purchases+Imports-Exports. This is the "guns versus butter" debate: every dollar the government spends on buying guns (or on government salary, buying pens for government use, etc) is one less dollar available for private consumption of butter. Though technically, since investment is also there, you could make it guns versus butter versus cows.