Money markets are very short duration debt securities, essentially the equivalent of cash traded between banks and offered to investors at a very nominal interest rate.
Money market securities are essentially IOUs issued by governments, financial institutions and large corporations, and they’re traded between each other in very high denominations. Retail investors can gain access to money markets via money market funds, which generally pay very low interest rates.
Banks and governments swap money markets in large quantities on a daily basis, to meet short-term debt obligations. Some examples of money market securities are short-term US Treasury bills, commercial paper, and Certificates of Deposit (CDs).