somethings in this thread are way far out. like we just keep printing money, we don't, it needs to be backed up by gold or bonds.
U.S. dollar is no longer backed by gold.
"Printing money" is a euphemism. Treasury prints the actual physical currency, but that is only a tiny fraction of the total money supply.
And yes, the Fed just creates it out of thin air.
My understanding:
1. When Fed wants to "create" money they call up one of their primary dealers (mega-banks, Goldman's, ect) and tells them the Fed would like to "buy" a trillion dollars of U.S. treasuries.
2. The bank says OK and transfers the assets they are authorized to sell.
3. The Fed says thanks and types in some numbers that just appear in the banks reserve account. They could have picked 2 trillion or 3 trillion. or 100 trillion. They just make up what ever they want. It's simply a matter of typing more digits.
4. The bank then has excess reserves they can lend out for profit or buy assets, thus injecting that magically created digital money into the economy.
5. The Fed can then hold that government debt on their balance sheet.
6. The government either pays it off from tax revenue or issues more debt to cover it.
The Fed "prints" money by telling the primary dealer how much government debt they want to buy. They just make that number up out of thin air depending on how much liquidity they think they want to inject into the economy. There is no limit other than the Fed's discretion.
New "money" is created in exchange for new government debt. How much money/debt is completely at the arbitrary discretion of the Fed.
[Edit] Sorry, I think the above is how QE works. And to add insult to injury, they are also buying mortgage backed securities. Private citizen's home loans. And other central banks (Japan?) are even buying private company stocks. Insane.
Here is a better description of the normal Fed mechanism:
https://www.investopedia.com/articles/investing/081415/understanding-how-federal-reserve-creates-money.asp 
