The notional value of each contract is US$50 times the value of the S&P 500 stock index, which means that an investor holding one of these contract will profit (or lose) $50 for every 1 point change on the S&P 500 index. When the S&P 500 is at 1000, each of these contracts is worth $50*1000 = $50,000. An investor does not have to pay the entire amount to hold these contracts; CME guidelines suggest that an investor needs anywhere between 5%-20% as collateral for these contracts.
Unlike ETFs, such as the SPDR funds, the e-mini futures are not backed by stocks. Hence an investor does not get the dividends on the underlying stocks. Instead, these contracts are backed by a counter-party who takes the opposite position on these futures.
Launched in 1997, these contracts have become extremely popular with investors. As of December 2004, on average over 700,000 of these contracts were traded every day. Other E-mini contracts include futures on the NASDAQ 100 and the Russell 2000 indices.