Exchange Traded Funds.
ETF's (Exchange Traded Funds) are traded on the stock market just like shares. The underlying difference is that while a share is a piece of a company an ETF is a piece of another "entity" such as a commodity. You can get ETFs that track things such as the Copper price which means you can invest in the price of copper without actually buying the metal and storing it. ETFs are available for entire sectors of industry, there are some that invest in everything to do with paper including timber and storage and others invest in diamond mines, transportation, and retail diamond stores.
You buy and sell ETFs the same way you buy and sell shares, on the stock market. Some ETFs are targeted towards short time ownership while others are targeted towards the buy and hold type of investor. They provide a convenient way to invest in sectors of the economy that might otherwise be out of reach.