A type of exchange-traded investment product that must register with the SEC as either an open-end investment company (generally known as “funds”) or a unit investment trust. ETFs offer investors a way to pool their money in a fund that makes investments in stocks, bonds, or other assets and, in return, to receive an interest in that investment pool. ETF shares are traded on a national stock exchange.
Source: http://investor.gov/glossary/glossary_terms/exchange-traded-fund-etf#.Us2xH2RDuEg