The Internal Revenue Service (IRS) is a US government agency responsible for the collection of taxes and enforcement of tax laws in the US. The Federal tax return is to be sent to the IRS.
Important: The IRS and the State tax office are two completely different institutions this is why when mailing your Federal tax return, you should send it to the IRS, and when mailing your State tax return, you should send it to the state tax office of the state you are filing for.