Money markets mainly involve short term borrrowing and lending attributed to the short maturities of the securities (usually a year or less). The main players are financial institutions and large companies which are able to borrow or lend large amounts that are well over $5 million in just one transaction. The biggest purpose of money markets is to help in supporting trade.
Financial instruments such as Certificates of Deposit, Treasury Bills, commercial papers amomg others, are characterized by short maturity, and high liquidity. This is precisely what makes them a good choice for short term loans. The short term lending and borrowing using these instruments is what is referred to as the money market. It can only happen however between financial institutions and banks, seeing as the trades are whole sale, and require large denominations which makes it a little difficult for individuals to trade in money markets. Individuals however can be given opportunities by the financial institutions to invest in the these assets.