Exchange Traded Fund (ETF)
Logically similar to an index fund - somebody somewhere is managing a stock portfolio that tracks an index - but it's traded on a stock exchange and you buy shares through a broker.
An ETF has the disadvantage that you pay broker's fees; it may have tax advantages.
But the greatest advantage may be convenience: if you already have a brokerage account then you are ready to buy shares of an ETF, using its symbol, the same way you would buy shares of a stock.
See the index funds article, especially this
tiny list of useful ETFs and index funds.