Money market funds primarily invest in short-term securities like Treasury bills.  Most money market funds offer a higher interest rate than a savings account at a bank.  However they are not FDIC insured like bank savings account are.  Some money market funds are free of taxes and others are not.  These funds are considered more stable investments than stock or bond funds.  There are two main type of money market funds, taxable which are securities of corporations and tax-free which are those of governments.