S&P 500 Index
Description: The S&P 500 index tracks the performance of some of the largest, most well-established U.S. companies. It is widely reported proxy for the performance of the market; although it has some flaws and there are better indexes, such as the Russell 1000 index (below).
Russell 1000 Index
Description: A much more academically-sound index in it’s construction, the Russell 1000 index is a subset of the larger Russell 3000 index representing the largest U.S. companies.
Russell 2000 Index
Description: the Russell 12000 index is a subset of the larger Russell 3000 index representing the smallest U.S. companies.