In short, a share is an ownership interest in a company and when you own shares you therefore become part owner of the company. In order for you to be able to buy shares in a company, the company needs to be a so-called limited company and the shares need to be listed on a stock exchange or a trading platform. The reason why companies choose to issue shares is to finance the company, either when the company is formed or when they feel that new capital needs to enter the company.