An expense ratio is the cost of operating a mutual fund, and it is always expressed as an annual percentage of the assets that are invested in the fund. For example, if a mutual fund has $200 million in assets and takes in $2 million in fees, its expense ratio is 1%. A sales load is the commission you pay when you buy and sell shares in the fund, and as such, it is a one-time fee. Some mutual funds don't charge sales commissions; they're known as no-load mutual funds.