ETFs invest in a wide array of securities, and which ones depends on the goal, strategy, or index that the ETF is built around. ETFs hold baskets of individual securities, of which investors can purchase an undivided interest in the form of ETF shares.
ETFs can be a good option if you want quick diversification, and there is an increasingly diverse selection of ETFs on the market. Many investment companies have issued new ETFs in the last 10 years. One of the biggest issuers is Barclays - one of the largest investment banks in the world — through their ETF brand iShares.
ETFs can give you a relatively cheap exposure to gold (GLD), oil (USO), the Brazilian Stock Market (EWZ), the Chinese Stock Market (FXI), to be bearish on technologies (QID), to be bearish on the entire stock market (SDS), to get exposure in a wide range of biotech companies (PBE), or to take a position reflecting your opinion that the price of agricultural products (DBA) will go up significantly.
ETFs also make more aggressive use of derivatives such as options and futures, compared to mutual funds.