What is the difference between a US Note and a Federal Reserve Note?

Federal Reserve Notes are obligations of the Federal Reserve System, the US central bank. The "Fed" is technically independent of the government so these bills are backed by the faith and credit of the bank rather than the US itself.

United States Notes were obligations of the federal government itself. Like Federal Reserve Notes they are "Fiat" money, but are backed by the faith and credit of the government rather than the central bank.

In practice the two forms of currency were effectively interchangeable, so in the later 1960s the government stopped printing US Notes. Since then all US paper money has been issued through the Federal Reserve System.

Starting with the 1928 series, both types of bills used almost identical designs. The most obvious difference is that US Notes had red seals and serial numbers while Federal Reserve Notes use the familiar green color. Also, Federal Reserve Notes have an indication (number and/or letter) of which Fed district distributed them. That feature was absent on US Notes because they weren't associated with any Fed districts.