Mutual fund
A pooled fund that invests many people's money in a single portfolio and prices its shares once a day, after the market closes, at net asset value.
A mutual fund pools money from many investors and runs one portfolio with it. Each shareholder owns a slice of the whole portfolio rather than any particular holding. Orders do not fill at the price showing when you place them. They fill at the net asset value struck after the close, which is the value of the holdings minus liabilities, divided by the shares outstanding.
Funds split into two broad kinds. An active fund pays people to choose holdings and charges more for the attempt. An index fund follows a published rule and charges less. Some funds add a sales charge, called a load, when you buy or when you sell, and one portfolio often comes in several share classes at different costs, so the expense ratio is worth reading per class rather than per fund.
The detail that catches people out, in the United States, is the yearly capital gains distribution. A fund passes its realised gains on to shareholders to keep its pass-through tax treatment, so you can owe tax in a taxable account without selling a share, in a year the fund itself fell. Held inside a tax-advantaged retirement account the problem goes away, which is why the same fund can behave quite differently in two accounts.