This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book focuses on 'equities' and the market in the UK for risk capital of 'listed' companies. It addresses the companies whose shares are traded on the London Stock Exchange (LSE) and its subsidiary market, known as the Alternative Investment Market (AIM) for smaller, growing companies. AIM's membership and reporting rules are less onerous than for the main market. Equities are sometimes referred to as 'ordinary' or 'common' shares or stock. There is no difference between a stock and a share, nor is there a difference between ordinary shares or common stock. These are also referred to as 'securities', a wider term to include stocks, shares and bonds. The book explains certain equity 'derivatives': that is options to buy or sell equities, spread betting on equities and contracts for difference (CFD) on equities.