A government shutdown starting Tuesday, Oct. 1, is now upon us. The House and Senate couldn't agree on a bill to fund the government, and time has run out.

So... it's shutdown time. Let's take a look at how this will work.

Not all government functions will simply evaporate come Oct. 1 — Social Security checks will still get mailed, and veterans' hospitals will stay open. But many federal agencies will shut their doors and send their employees home, from the Environmental Protection Agency to hundreds of national parks.

Here's a look at how a shutdown will work, which parts of the government will close, and which parts of the economy might be affected.

Wait, what? Why is the federal government on the verge of shutting down?

Short answer: There are wide swaths of the federal government that need to be funded each year in order to operate. If Congress can't agree on how to fund them, they have to close down. And, right now, Congress can't agree on how to fund them. To get a bit more specific: Each year, the House and Senate are supposed to agree on 12 appropriations bills to fund the federal agencies and set spending priorities. Congress has become really bad at passing these bills, so in recent years they've resorted to stopgap budgets to keep the government funded (known as "continuing resolutions"). The last stopgap passed on March 28, 2013, and ends on Sept. 30. In theory, Congress could pass another stopgap before Tuesday. But the Democratic-controlled Senate and Republican-controlled House are at odds over what that stopgap should look like. The House passed a funding bill over the weekend that delayed Obamacare for one year and repealed a tax on medical devices. The Senate rejected that measure. They voted a few more times and still no agreement. So... we're getting a shutdown.

Does a shutdown mean everyone who works for the federal government has to go home?

Not exactly. The laws and regulations governing shutdowns separate federal workers into "essential" and "non-essential." (Actually, the preferred term nowadays is "excepted" and "non-excepted." This was tweaked in 1995 because "non-essential" seemed a bit hurtful. But we'll keep things simple.) The Office of Management and Budget recently ordered managers at all federal agencies to conduct reviews to see which of their employees fall into each of these two categories. If a shutdown hits, the essential workers stick around, albeit without pay. The non-essential workers have to go home after a half-day of preparing to close shop.

Which parts of government stay open?

Air traffic control stays open. There are a whole bunch of key government functions that carry on during a shutdown, including anything related to national security, public safety, or programs written into permanent law (like Social Security). Here's a partial list: -- Any employee or office that "provides for the national security, including the conduct of foreign relations essential to the national security or the safety of life and property." That means the U.S. military will keep operating, for one. So will embassies abroad. -- Any employee who conducts "essential activities to the extent that they protect life and property." So, for example: Air traffic control stays open. So does all emergency medical care, border patrol, federal prisons, most law enforcement, emergency and disaster assistance, overseeing the banking system, operating the power grid, and guarding federal property. -- Agencies have to keep sending out benefits and operating programs that are written into permanent law or get multi-year funding. That means sending out Social Security checks and providing certain types of veterans' benefits. Unemployment benefits and food stamps will also continue for the time being, since their funding has been approved in earlier bills. -- All agencies with independent sources of funding remain open, including the U.S. Postal Service and the Federal Reserve. -- Members of Congress can stick around, since their pay is written into permanent law. Congressional staffers however, will also get divided into essential and non-essential, with the latter getting furloughed. Many White House employees could also get sent home.

Do these "essential" employees who keep working get paid?

The 1.3 million or so "essential" civilian employees who stay on could well see their paychecks delayed during the shutdown, depending on the timing. They should, however, receive retroactive pay if and when Congress decides to fund the government again. The 1.4 million active-service military members, meanwhile, will get paid no matter how long the shutdown lasts. That's because the House and Senate specifically passed a bill to guarantee active-duty military pay even when the government is closed. Obama signed it into law Monday night.

So which parts of government actually shut down? 

Everything else, basically. It's a fairly long list, and you can check out in detail which activities the agencies are planning to halt in these contingency plans posted by each agency. Here are a few select examples:

Health: The National Institutes of Health will stop accepting new patients for clinical research and stop answering hotline calls about medical questions. The Centers for Disease Control and Prevention will stop its seasonal flu program and have a "significantly reduced capacity to respond to outbreak investigations."

Housing: The Department of Housing and Urban Development will not be able to provide local housing authorities with additional money for housing vouchers. The nation's 3,300 public housing authorities will also stop receiving payments, although most of these agencies have enough cash on hand to provide rental assistance through the end of October.

Immigration: The Department of Homeland Security will no longer operate its E-Verify program, which means that businesses will not be able to check on the legal immigration status of prospective employees during the shutdown.

Law enforcement: Although agencies like the FBI and the Drug Enforcement Agency will continue their operations, the Justice Department will suspend many civil cases for as long as the government is shut down.

Parks and museums: The National Park Service will close more than 400 national parks and museums, including Yosemite National Park in California, Alcatraz in San Francisco, and the Statue of Liberty in New York. The last time this happened during the 1995-96 shutdown, some 7 million visitors were turned away. (One big exception was the south rim of the Grand Canyon, which stayed open only because Arizona agreed to pick up the tab.)

Regulatory agencies: The Environmental Protection Agency will close down almost entirely during a shutdown, save for operations around Superfund sites. Many of the Labor Department's regulatory offices will close, including the Wage and Hour Division and the Occupational Safety and Health Administration. (The Mine Safety and Health Administration will, however, stay open.)

Financial regulators. The Commodity Futures Trading Commission, which oversees the vast U.S. derivatives market, will largely shut down. A few financial regulators, however, such as the Securities and Exchange Commission, will remain open. (Small parts of) Social Security: The Social Security Administration will retain enough staff to make sure the checks keep going out. But the agency won't have enough employees to do things like help recipients replace their benefit cards or schedule new hearings for disability cases.

Visas and passports: The State Department says it will keep most passport agencies and consular operations open so long as it has the funds to do so, although some activities might be interrupted. (For instance, "if a passport agency is located in a government building affected by a lapse in appropriations, the facility may become unsupported.") During the previous shutdown in 1995-1996, around 20,000 to 30,000 applications from foreigners for visas went unprocessed each day. This time around, the State Department is planning to continue processing visas through the shutdown, since those operations are largely funded by fees collected.*

Veterans: Some key benefits will continue and the VA hospitals will remained open. But many services will be disrupted. The Veterans Benefits Administration will be unable to process education and rehabilitation benefits. The Board of Veterans' Appeals will be unable to hold hearings. What's more, if the shutdown lasts for more than two or three weeks, the Department of Veterans Affairs has said that it may not have enough money to pay disability claims and pension payments. That could affect some 3.6 million veterans. Rep. Rush Holt (D-N.J.) has a list of other possible effects of a shutdown. Funds to help states administer unemployment benefits could get disrupted, IRS tax-refund processing for certain returns would be suspended, farm loans and payments would stop, and Small Business Administration approval of business loan guarantees and direct loans would likely cease.

Read it at The Washington Post.