Exchange Traded Funds (or ETFs) are investment funds with a lot in common with mutual funds. Instead of being managed by a professional Fund Manager, ETFs are designed to replicate the movement of some other index, such as the S&P 500, or follow the price of oil. These can be used both to diversify a portfolio and to invest in things like commodities, which do not normally trade on the stock exchange.
ETFs are a good way to invest in a broad range of investments all at once, such as broadly investing in Biotech stocks. They can also be used to “Go Short” on a broad index, or even use a Leveraged ETF to double the gains (or losses) of whatever it is tracking!