The FTSE 100 Index - also known as the Financial Times Stock Exchange or the “Footsie” - tracks the performance of the largest 100 stocks by market capitalization traded on the London Stock Exchange.
Since many of these companies are multinationals, however, the FTSE 100 does not provide the best gauge for how the UK economy is performing. The FTSE 225 is a better barometer for the UK.
The FTSE 100, however, much like the S&P 500 provides a good general gauge for how stocks are performing globally.