What is the Internal Revenue Service?

I was watching the Shawshank Redemption when they were tarring the roof and Andy tried talking to the Guard how he can get around the IRS with his 35,000 dollars. I was also watching Catch Me If You Can, in the beginning how Franks Father was trying to get a business loan but due to unspecified difficulties with... show more I was watching the Shawshank Redemption when they were tarring the roof and Andy tried talking to the Guard how he can get around the IRS with his 35,000 dollars.

I was also watching Catch Me If You Can, in the beginning how Franks Father was trying to get a business loan but due to unspecified difficulties with the IRS wasn't allowed.

And it just makes me wonder...what is the IRS?
Update: Is the IRS world wide or just the United States?
Because I only hear about it in American films and never here in real life where I live in the UK.
Update 2: Is the IRS world wide or just the United States?
Because I only hear about it in American films and never here in real life where I live in the UK.
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