Every company that manages a mutual fund charges an annual fee - generally 0.5% to 2.5% of assets - as well as certain other expenses.
In addition, some funds slap you with a sales charge over and above those fees. Those funds are called "load funds." (Funds that do not impose sales charges are known as "no-loads.") Those sales charges are either a cut of new money you put into the fund, or a cut of withdrawals you make from the fund.