Shutdown - All or part of the Federal government closes for normal business.  It affects whole agencies or major parts.

Only Congress may authorize the use of money.  Agency operations involve the use of money.  If Congress has not authorized the use of money, then the agency may not operate and therefore closes down.  (See TO CONGRESS for more details.)
If for some reason there is no appropriations act or a continuing resolution by the start of a fiscal year, then the agency must cease operations.
Reasons for failing to act on an appropriation (either for the full year or a continuing resolution) are related to policy differences between Congress and the President, and sometimes policy differences among members of Congress.
At the beginning of the fiscal year, usually October.  May extend for months if short-term continuing resolutions (CR) are passed as stop-gaps since the end of each CR can result in a shutdown if there is not new CR or an annual appropriations act.
Documents and Links
Shutdowns are essentially current events, resulting from the failure of normal processes of government.  For links and additional insights, see (or return to) the Analyst's OPINIONS on the matter.