ETFs are securities that track various indexes such as the Dow Jones Industrial Average, the S&P 500, the S&P Healthcare sector, gold, oil, or currencies. They “trade” like stocks (with price fluctuations throughout the day) and allow investors to buy (and sell) throughout the trading day (ex. NYSE open 9:30am to 4:30pm EST). They are sort of the next step in evolution to index mutual funds (which do not trade throughout the day). The key difference is that the investor can pick his/her price point to buy or sell the index, giving the investor more control over what price s/he pays for the respective index.

some definitions from Google:
# exchange traded fund: a mutual fund that is traded on a stock exchange
wordnet.princeton.edu/perl/webwn

# A security that tracks an index and represents a basket of stocks like an index fund, but trades like a stock on an exchange, thus experiencing …
investors.redhat.com/glossary.cfm

# Exchange traded funds are sort of like mutual funds that trade on an exchange like stocks. They are funds that own other securities, and therefore, when you buy an ETF, you indirectly own whatever the ETF owns.
www.universalwealthcreation.com/stockmarket/glossary-3.html

# An investment fund, units of which can be bought and sold on a stock exchange. Often used by tracker funds….more on ETF
moneyterms.co.uk/d/

# Collections of stocks that are bought and sold as a package on an exchange, principally the American Stock Exchange, but also the NYSE, CBOE, and Nasdaq.
stockcharts.com/school/doku.php

# A collective investment vehicles which track indices – they can allow low cost exposure to the performance of an index as quickly and efficiently as the most liquid UK stocks
www.halifax.co.uk/sharedealing/Education_Centre/Glossary.asp

# Exchange traded funds are a relatively new technique. Instead of buying a unit in a tracker fund, investors can “buy” an index in the …
www.ubs.com/1/e/gcc/bankingterms.html

# a basket of securities that tracks the performance of a stock, bond, or commodity index. Similar to stock, each ETF has a ticker and is traded on stock exchange. >> more
bankernotes.blogspot.com/2006/10/financial-term-glossary.html

# (Early Termination Fee) This fee is usually anywhere from $150 to $200 depending on state laws and the time that you signed up your service. It is the penalty fee enforced by the carrier when the contract is ended early by the user either voluntarily or by nonpay.
wiki.howardforums.com/index.php/Cingular_Acronyms_and_Definitions

# An Exchange Traded Fund is essentially an “open ended fund’ traded on an exchange. Most major stock exchanges now list a number of ETFs. Historically ETFs were index based in that the assets they held reflected the make-up of a particular “index”, for example the Dow Jones Industrial Average. …
www.my-worth.com/doc/glossary/

ETF Center at Yahoo

http://finance.yahoo.com/etf/browser/mkt?c=0&k=3&f=0&o=a&cs=1&ce=838