Answer:
Federal Reserve Notes are non-interest bearing promissory notes issued by the US Federal Reserve System (the central bank) and are official Legal Tender for payment of debts.
Since 1966 all US bills have been issued by the Federal Reserve System, identified by the bills' green seals. Current denominations are $1, $2, $5, $10, $20, $50, and $100. Up till 1969 bills were also available in denominations of $500, $1,000, $5,000 and $10,000 but these rarely circulated.
In the past, other bills were issued directly by the Federal Government. These included blue-seal silver certificates, gold-seal gold certificates, and red-seal United States Notes. Gold and silver certificates were backed $1-for-$1 with bullion on deposit with the US Treasury, while US Notes were promissory notes very similar to Federal Reserve Notes.
In the 19th and early 20th centuries, private banks also issued bills under Federal authority.