A stock index is a group of stocks that can be bought or sold as a single tradable instrument. Now, some traders speculate on how the price of a single asset changes, and some choose to speculate with Stock Indices. As a group, stock indices can be used to indicate the health of an industry or even a country.

Classifying stock indices, however, is a little more complex. Some indices, like the DAX 30 for example, is a group of the 30 top-performing companies in Germany. Classified as a 'national stock index' it gives an indication of the health of the German stock market.

On the other hand, stock indices aren't only comprised of stocks that are grouped together because of their geographical location. Some stock indices represent and track the performance of certain sectors of the market. For example, the Nasdaq 100 index, tracks the performance of all the companies listed on the Nasdaq exchange. Generally being technology-related firms, the Nasdaq gives an indication of the health of the technology sector in the US.