Exchange-traded funds, or ETFs, are investment companies that are legally classified as open-end companies or unit investment trusts (UITs), but that differ from traditional open-end companies and UITs. The main differences are that ETFs do not sell directly to investors and they issue their shares in what are called "Creation Units" (large blocks such as blocks of 50,000 shares). Also, the Creation Units may not be purchased with cash but a basket of securities that mirrors the ETF's portfolio. Usually, the Creation Units are split up and re-sold on a secondary market.