Unlike index ETFs, which are backed by stocks in an index, these contracts are backed by a counter-party taking an opposite position on the contract. For example: If investor A buys an e-mini contract on the S&P 500, and investor B shorts an e-mini contract on S&P 500, it means that investor B agrees to pay investor A if the index rises and vice versa. Since these contracts are backed by cash, they allow investors to take a levered position on the index-- i.e. the investor in these contracts can buy them and put up from anywhere between 5% and 20% of the cash the investor would have to put up to invest in an index ETF, such as the SPDRs (the rest is covered by borrowed money).
Since they are not backed by stocks, investors give up dividend income by investing in these contracts.
These contracts are available through a licensed Series 3 commodity futures broker, and are open for trading throughout the week (even at nights), with the exception of Friday night to Sunday morning (Chicago Time).
Launched in 1997, these contracts have become popular. As of December 2004, on average over 700,000 of S&P 500 contracts and 300,000 NASDAQ 100 contracts were traded every day.
As of 2008, CME offered E-mini contracts on the following indices: