The IRS States:

Due to the current lapse in appropriations, IRS operations are limited. However, the underlying tax law remains in effect, and all taxpayers should continue to meet their tax obligations as normal.

Individuals and businesses should keep filing their tax returns and making deposits with the IRS, as they are required to do so by law. The IRS will accept and process all tax returns with payments, but will be unable to issue refunds during this time. Taxpayers are urged to file electronically, because most of these returns will be processed automatically.

No live telephone customer service assistance will be available, however most automated toll-free telephone applications will remain operational. IRS walk-in taxpayer assistance centers will be closed.

Explanation in Bureaucratic Terms:

IRS employees dealing with refunds are “non-essential employees” and those taking your money are “essential employees.”