What is a mutual fund?
A mutual fund is a type of investment that includes a well diversified pool of stocks across a broad range of industry verticals.  Many investors invest in mutual funds and usually stay in them for the long term.  It is a good idea to diversify a stock portfolio like a 401k with mutual funds that have annual percentage return on the average of about 10% - 12%.  This is great way to get a good return on your money without having to actively manage your brokerage account.