A money market fund is a package of government debt instruments that is assembled by a fund management company. The package is typically comprised of Treasury bills, notes, and bonds. The investment is highly liquid, with many investors putting in funds for as little as a day. The intent of a money market fund is to provide investors with a safe and highly liquid investment.
There are varying levels of risk between different money market funds, since some funds are more active in trying to outperform the market (with an attendant increase in risk).