Each agency is responsible for identifying its essential staff. Federal employees who are "necessary to protect life and property" and are needed to perform an "orderly shutdown of emergency operations" are considered "essential." That includes most national intelligence staff, military personnel, air traffic controllers, law enforcement, emergency and disaster personnel, the Transportation Security Administration, Coast Guard and similar staff.
During the last full five-day shutdown in 1995, an estimated 800,000 federal employees were furloughed, according to the Congressional Research Service. A smaller figure, 284,000, were furloughed in the partial 21-day shutdown that followed soon after.
Amid contractors, who are unlikely to receive back pay, more than 20 percent were negatively impacted by the last funding lapse.
A much larger number likely will be affected this time because of the size and scope of the federal government.
Washington, D.C. Mayor Vince Gray said Tuesday that because D.C.'s federal subsidy would be affected, trash collection and pothole repair in the city could be threatened during a shutdown.
The ripple effects of a shutdown will be felt outside of the nation's capital. The U.S. Postal Service will operate as normal, since it is self-funded. Social Security, veterans and Medicare checks would continue to be disbursed, although there could be a delay in services for new registrants and those who have filed a change of address form.
Many Americans may have to hold off on their travel plans. Museums and national parks will close, as will the national zoo, and passport applications will be delayed.
Some government inspection services, such as for meat, may be delayed.
The uncertainty also could roil stock markets, rattle consumer confidence and hurt tourism, with the severity depending on how long a shutdown lasts.
The average federal government worker makes $1,404 weekly, according to the latest U.S. Bureau of Labor Statistics figures. If 800,000 of them are furloughed and don't get a paycheck during a government shutdown, it zaps about $1.1 billion out of the economy in direct employee compensation each week.
ABC News' Barbara Garcia, Jon Karl, Dan Arnall and Jake Tapper contributed to this report.