Stock Indices use a portfolio of representative companies (usually spanning major industries) to reflect the status of the whole stock market. There are basically three kinds: global, regional and national. Global indices include companies regardless of where they are traded. Regional indices include companies from a certain region and national indices include companies from a specific nation. Stock Indices are used to get an indication of the market's overall direction. Some analysts use them as a barometer of the underlying economy.
Indices can be comprised of tens to hundreds of stocks and each index calculates the weighted average differently. Some weigh the stocks equally (equal weighting), others take company size into account (capitalization weighting) and others use a hybrid method (modified capitalization weighting). Stock Indices are tradable entities themselves. A currency index is a measure of the value of a specific currency relative to other select currencies. Indices like the US Dollar Index or the Euro Currency Index are used to gauge the strength of those respective currencies.