An exchange-traded fund (ETF) is a fund (consisting of a collection of assets like stocks, commodities and/or bonds) that is traded (bought and sold) on a stock exchange like the NYSE or NASDAQ.
An ETF can be bought and sold anytime the exchange (e.g. NYSE) is open. This means its price can fluctuate when the exchange is open. This contrasts with a mutual fund, which is typically only bought/sold at one fixed time during the day. ETFs typically, but not always, have lower fees and better tax efficiency than mutual funds.
Wikipedia's article on exchange-traded funds
Investopedia's article on ETFs