The FTSE 100 is an index of the 100 companies with the highest market capitalisation on the London Stock Exchange. Although, many of the listed companies are international, making it a somewhat weak indicator of the UK economy.
The FTSE 250 is an index of the next largest 250 companies based on market capitalization after the FTSE 100, and the FTSE 350 is a combination of both indices. These other indices are seen as better indications of the UK economy as they contain a smaller proportion of international companies.
FTSE stands for ‘Financial Trade Stock Exchange’. Informally the FTSE 100 is called the ‘footsie.’ The FTSE group oversees the multiple FTSE indices, and is itself wholly owned by the London Stock Exchange.