Only so-called authorized participants (typically, large institutional investors) actually buy or sell shares of an ETF directly from/to the fund manager, and then only in creation units, large blocks of tens of thousands of ETF shares, which are usually exchanged in-kind with baskets of the underlying securities. Authorized participants may wish to invest in the ETF shares long-term, but usually act as market makers on the open market, using their ability to exchange creation units with their underlying securities to provide liquidity of the ETF shares and help ensure that their intraday market price approximates to the net asset value of the underlying assets. Other investors, such as individuals using a retail broker, trade ETF shares on this secondary market.

An ETF combines the valuation feature of a mutual fund or unit investment trust, which can be bought or sold at the end of each trading day for its net asset value, with the tradability feature of a closed-end fund, which trades throughout the trading day at prices that may be more or less than its net asset value. Closed-end funds are not considered to be "ETFs", even though they are funds and are traded on an exchange. ETFs have been available in the US since 1993 and in Europe since 1999. In 1993, the first country specific ETFs were a collaboration between MSCI, BGI and a small independent third party Distribution firm called Funds Distributor, Inc. The product eventually evolved into the iShares brand widely known around the globe. ETFs traditionally have been index funds, but in 2008 the U.S. Securities and Exchange Commission began to authorize the creation of actively managed ETFs.

ETFs offer public investors an undivided interest in a pool of securities and other assets and thus are similar in many ways to traditional mutual funds, except that shares in an ETF can be bought and sold throughout the day like stocks on a securities exchange through a broker-dealer. Unlike traditional mutual funds, ETFs do not sell or redeem their individual shares at net asset value, or NAV. Instead, financial institutions purchase and redeem ETF shares directly from the ETF, but only in large blocks, varying in size by ETF from 25,000 to 200,000 shares, called "creation units". Purchases and redemptions of the creation units generally are in kind, with the institutional investor contributing or receiving a basket of securities of the same type and proportion held by the ETF, although some ETFs may require or permit a purchasing or redeeming shareholder to substitute cash for some or all of the securities in the basket of assets.

The ability to purchase and redeem creation units gives ETFs an arbitrage mechanism intended to minimize the potential deviation between the market price and the net asset value of ETF shares. Existing ETFs have transparent portfolios, so institutional investors will know exactly what portfolio assets they must assemble if they wish to purchase a creation unit, and the exchange disseminates the updated net asset value of the shares throughout the trading day, typically at 15-second intervals.

If there is strong investor demand for an ETF, its share price will (temporarily) rise above its net asset value per share, giving arbitrageurs an incentive to purchase additional creation units from the ETF and sell the component ETF shares in the open market. The additional supply of ETF shares increases the ETF's market capitalization and reduces the market price per share, generally eliminating the premium over net asset value. A similar process applies when there is weak demand for an ETF and its shares trade at a discount from net asset value.

In the United States, most ETFs are structured as open-end management investment companies (the same structure used by mutual funds and money market funds), although a few ETFs, including some of the largest ones, are structured as unit investment trusts. ETFs structured as open-end funds have greater flexibility in constructing a portfolio and are not prohibited from participating in securities lending programs or from using futures and options in achieving their investment objectives.

Under existing regulations, a new ETF must receive an order from the Securities and Exchange Commission, or SEC, giving it relief from provisions of the Investment Company Act of 1940 that would not otherwise allow the ETF structure. In 2008, however, the SEC proposed rules that would allow the creation of ETFs without the need for exemptive orders. Under the SEC proposal, an ETF would be defined as a registered open-end management investment company that:

The SEC rule proposal would allow ETFs either to be index funds or to be fully transparent actively managed funds. Historically, all ETFs in the United States have been index funds. In 2008, however, the SEC began issuing exemptive orders to fully transparent actively managed ETFs. The first such order was to PowerShares Actively Managed Exchange-Traded Fund Trust, and the first actively managed ETF in the United States was the Bear Stearns Current Yield Fund, a short-term income fund that began trading on the American Stock Exchange under the symbol YYY on 25 March 2008. The SEC rule proposal indicates that the SEC may still consider future applications for exemptive orders for actively managed ETFs that do not satisfy the proposed rule's transparency requirements.

Some ETFs invest primarily in commodities or commodity-based instruments, such as crude oil and precious metals. Although these commodity ETFs are similar in practice to ETFs that invest in securities, they are not "investment companies" under the Investment Company Act of 1940.

Publicly traded grantor trusts, such as Merrill Lynch's HOLDRS securities, are sometimes considered to be ETFs, although they lack many of the characteristics of other ETFs. Investors in a grantor trust have a direct interest in the underlying basket of securities, which does not change except to reflect corporate actions such as stock splits and mergers. Funds of this type are not "investment companies" under the Investment Company Act of 1940.