The FTSE All-Share Index, originally known as the FTSE Actuaries All Share Index, is a stock market index which aims to capture 98% of the full value of the London Stock Exchange. This means it is an accurate reflection of the overall stock market.
The index is a capitalisation-weighted index. This means that companies are given a weighting depending on their market capitalisation. i.e. movements in the biggest companies will have much more effect than the smallest companies
The FTSE All share index includes around 800 of more than 2,000 companies traded on the London Stock Exchange.