Mutual funds date back to the 18th century, when funds were typically closed-end—meaning shares were only sold to a limited number of investors. Most modern mutual funds, developed during the 20th century, are open-end, which means anyone can buy in or sell out at any time.
Mutual fund shares don't trade on an exchange, like a stock does. Instead, they trade at the end of the day. First, the closing prices of the underlying assets are used to calculate the net asset value (NAV) of the fund's holdings. This figure is divided by the number of shares (less liabilities) to calculate the price per share of the fund. Once the price is set, the fund can issue shares to investors looking to buy, and redeem shares for people looking to sell.