Today's Washington Post has the article Congress, Obama Brace for Showdown as Government Shutdown Looms, which contains the following sentence:
The prospect of a government shutdown appeared more possible Saturday after the House passed a budget measure in the pre-dawn hours that cuts $61 billion - and was immediately rejected by Senate Democrats and President Obama.
What does government shutdown mean? Unlike stores and factories, or Greece and Iceland, how can the government of America shut down, or get paralyzed with its operation? I know it's a naive question, but I don't understand a government can stop its function in reality.