The IRS is the Internal Revenue Service, the United States government office that collects national taxes. The two big ones people mean are income tax, based on how much a person earns, and corporation tax, based on how much profit a company makes. People in the US have to send an income tax return in April each year: an official form showing tax already paid and tax still owed. In American news and films the IRS often feels a little feared; a letter from them can mean a check of your numbers, called an audit. It is a US office, not a world tax police. Though actually, other countries have their own tax offices with other names, so IRS is not a generic word for any tax bureau.