A shutdown occurs when the Federal Government runs out of money.
The U.S. Government budget routinely contains funds that run through midnight of October 1. If Congress does not approve and the President does not sign bills approving new funding, then certain functions will not continue and certain people will not get paid as they usually do.
During the last shutdown, about 800,000 workers were initially put on leave. (Their wages were restored to them retroactively.)
Many federal employees will be put on temporary leave, meaning that they won't get paid for the time that they're not working, because they agencies they work for will not be open for business.
Most Government Departments have contingency plans, including everything from laying off employees to closing facilities. National Parks are usually the most prominent of the closings. Processes delayed usually include visa applications. NASA operations will be shut, except for vital systems supporting the crew currently aboard the space station.
Among the federal agencies or groups that will still be open for business because their operations have been deemed too essential to be suspended are the following:
Veterans will still get their veteran benefits, and retirees will still get their allotments from Social Security, Medicare, and Medicaid. Members of Congress will get paid but only retroactively, as part of a bill that reactivates Government funding.
The U.S. Government has shut down 17 times. The first shutdown, in 1976, lasted 10 days. Most shutdowns lasted no more than five days. The longest shutdown, in 1995 and 1996, lasted 21 days.