A balance sheet gives a snapshot of a corporation’s assets and liabilities. The difference between total assets and total liabilities is (by definition) the value of the equity owned by the owners or shareholders of the company.
With, say, a manufacturing firm, the assets would include tangible items such as buildings and equipment and inventory, and intangibles such as cash, bank accounts, and accounts receivable. Liabilities may include mortgages and other loans, and accounts payable such as taxes, wages, pensions, and bills for purchased goods.
The balance sheet for a bank is different. The “Assets” are mainly loans that the bank has made, plus some securities (such as US Treasury bonds) that the bank has purchased. These assets pay interest to the bank. The money the bank used to make these loans and purchase these securities came mainly from customer deposits or other borrowings by the bank (which are considered “Liabilities” of the bank), and also from paid-in capital from the bank owners/shareholders.  As usual, the current equity of the bank is assets minus liabilities. Thus:
The Federal Reserve System is a complex beast. We will not delve into all the components and moving parts, but just take a look at the overall balance sheet.
Unlike other banks, the Fed has the magical power of being able to create money out of thin air. Technically, what the Fed can do with that money is mainly make loans, i.e. buy interest-bearing securities such as government bonds. The Fed makes its transactions through affiliated banks, so it credits a bank’s reserve account with a million dollars, if it buys from that bank a million dollars’ worth of bonds. Those bonds then become part of the Fed’s “assets”, while the reserve account of the bank at the Fed (which is a liability of the Fed) becomes larger by a million dollars. Since the Fed is not a for-profit bank, the “Equity” entry on its balance sheet is nearly zero. Thus, total assets are essentially equal to total liabilities.
The Fed also has the power of literally printing money, in the form of Federal Reserve Notes (printed dollar bills). These, too, are classified as liabilities. Thus, you are probably carrying in your wallet right now some of the liabilities of the central bank of the United States.
Before 2008, the balance sheet of the Fed was under a trillion dollars. Nearly all the “Liabilities” were the Federal Reserve Notes and nearly all the “Assets” were US Treasury securities. The reserve accounts of the affiliated Depository Institutions was minuscule. All that changed with the Global Financial Crisis of 2008-2009. To help stabilize the financial system, the Fed started buying lots of various types of securities, including mortgage-backed securities (MBS) . The Fed thus propped up the value of these securities, and injected cash (liquidity) into the system.
Here is a plot of how the assets of the Fed ballooned in the wake of the GFC, from about $ 0.9 trillion to over $ 4 trillion:
The initial purchases in 2008 were US Treasuries, which the Fed had prior authorization to do. To buy other securities, especially the mortgage products, required congressional authorization. The increased liabilities of the Fed which offset these purchases were mainly in the form of larger reserve accounts of the affiliated banks. The Fed started paying interest on these reserve accounts, to keep short term interest rates above zero at all times (otherwise the whole money market in the U.S. might implode).
With the Fed relentlessly buying the mortgage and bond products, the interest rates on long-term mortgages and bonds was kept low. This was deemed good for economic growth. The Fed tried to sell off some securities to taper down its balance sheet in 2018, but that effort blew up in its face – – the stock market started crashing in response in late 2018, and so the Fed backtracked . You can look at weekly tables of the Fed balance sheet here.
Anyway, the GFC and its aftermath provided the precedent for massive purchases of “stuff” by the Fed. When the Covid shutdown of the economy hit in March of this year, the Fed very quickly went into high gear. Its balance sheet shot up from $4 trillion to $7 trillion in just a few months. It bought not only Treasuries and MBS, but corporate bonds. This was way outside the Fed’s original charter, but the crisis was so intense that nobody seemed to care whether these actions were legal or not. And now, to finance the huge deficit spending of the federal government in the wake of the shutdowns, the Fed has been buying up nearly the entire issuance of Treasury bonds and notes.
These actions may have long term consequences we will explore in later posts . For now, the Fed has made it clear that it will keep interests rates near zero for at least the next couple of years. Invest accordingly.
 Huge caveat: This statement gives the impression that a bank must first receive say a thousand dollar deposit before it can make a thousand dollar loan. That is not the case. The reality is just the opposite: the act of making a thousand dollar loan actually CREATES a corresponding thousand dollar deposit. This is very counterintuitive, and I won’t try to explain or justify this point here.
 Technically, the Fed is not “buying” the mortgage-backed security (MBS). Rather, it is making a “loan” to the bank, and holding the MBS as collateral against that loan.
 It is now harder to take the federal deficit seriously as a constraint on spending: the government can issue unlimited bonds to fund deficits, which the Fed will purchase to keep interest rates low. Yes, the government has to pay interest on those bonds, but the Fed has to return most of that interest to the Treasury, so the real cost to the government of that extra debt is low.