The securities market dealing in short-term debt and monetary instruments. Money market instruments are forms of debt that mature in less than one year and are very liquid.
Treasury bills make up the bulk of the money market instruments.
Securities in the money market are relatively risk-free. The Money Market - a tutorial by Investopedia on what the money market is and the different types of mopney market securities out there. Money Market Fund Basics - an excellent introduction to money market mutual funds. Investing in Money Market Securities - another great primer for money market instruments.