Our government does a lot of things, which is why the prospect of a shutdown to begin later this week can be so daunting and confusing. Basically, during a shutdown, federal agencies must freeze all non-essential services. How does each organization decide what's essential? And what does that word even mean to a government that rarely agrees on what's important?

A shutdown would be bad; hopefully, the budget battle doesn't come to that. Best to be prepared, so here's a rundown of how our government makes these difficult choices.

What's essential

In the event of a shutdown, every government agency is responsible for determining which jobs under their umbrella fit the following criteria for what's absolutely necessary to keep America running:

• Providing for the national security, including the conduct of foreign relations essential to the national security or the safety of life and property.
• Benefit payments and the performance of certain contract obligations
• Medical care of inpatients and emergency outpatient care
• Conducting activities to ensure continued public health and safety, including safe use of food, drugs and hazardous materials
• Air traffic control and other transportation safety functions
• Border and coastal protection and surveillance
• Protection of federal lands, buildings, waterways, equipment and other property owned by the United States
• Care of prisoners and other persons in the custody of the United States
• Law enforcement and criminal investigations
• Emergency and disaster assistance
• Activities that ensure production of power and maintenance of the power distribution system
• Activities essential to the preservation of the essential elements of the money and banking system of the United States, including borrowing and
  tax collection activities of the Treasury
• Activities necessary to maintain protection of research property.

Employees who get rated "essential" are still allowed to report to work, whereas everyone else is barred from performing their duties, even if they offer to do so for free. Essential personnel get paid, but only retroactively, once funding for their particular agency has been restored.

Despite these guiding principles, the process an agency goes through to in identifying these functions and the employees who perform them is a highly subjective task.

What's not

Since 1980, every government agency has been required to have a plan in the case of a shutdown, defining essential employees and operations. The Office of Management and Budget (OMB) mandated this in a memo that continues to be updated.

These plans are not available to the public, which is why there is so much speculation in advance of the expected fallout. We simply don't know what's considered essential these days. The last time the government shut down was in 1995; so much has changed with respect to the government's business and how it's conducted since then. In 1995, these services were not deemed essential by their parent agencies, and therefore went untended:

• Health, welfare and financial services for Veterans
• Passport and visa application processing
• National parks, museums and monument
• Work on bankruptcy cases
• Recruitment and testing of federal law-enforcement officials
• Disease surveillance and information distribution
• Toxic waste clean-up

It's a good bet that most things deemed unessential in 1995 are still unessential in 2011. There is speculation that this time around, however, the SEC may have to put off audits and interviews, and that the decision of which cybersecurity personnel to keep on would not only be especially delicate, but could also open the door to attacks from hackers who are more numerous and better organized than they were 15 years ago.