Money market funds are mutual funds. However, they don't invest in stocks – they invest in U.S.
Treasury bills (notes and bonds with less than 13 months until maturity), top-rated commercial paper, bank notes, and other high-quality, short-term debt instruments. (They shouldn't to be confused with money market accounts, or money market deposit accounts, which are FDIC-insured but pay much lower rates on average). Money market funds attempt to maintain a stable dollar per share NAV (net asset value), so their risk exposure is non-existent when compared with stock and bond funds. Money market funds are not insured or guaranteed by the government (or anyone else), but the Securities and Exchange Commission heavily regulates them; they have a number of restrictions on the quality, maturity and diversity of the securities they may invest in.