A mutual fund is an entity that is registered with the Securities & Exchange Commission whose purpose is invest and trade securities for a group of people who will share in the dividends or income earned from interest and who, after management fees, will distribute all trading profits/losses and/or investment gains/losses to the company's holder.
So it's a company that pools investors’ monies for the purposes of buying and selling securities to make trading profits and/or collect dividends and interest in which the investors will share in.
For this service, the fund company will charge a fee and this fee will be deducted before they will distribute any monies to the holders
You can go to MSN Money,
http://moneycentral.msn.com/home.asp they have an entire section about mutual funds. While you're there you should also familiarize yourself with ETF (exchange traded funds)
You could also check out
http://www.brokerage101.com/ for general information and then visit Morningstar which is a firm that rates mutual funds
http://www.morningstar.com/
I hope I've been of some help or at least have given you proper direction. Study well and invest even better