An initial public offering, or IPO, is the first sale of a corporation's common shares to investors on a public stock exchange. The main purpose of an IPO is to raise capital for the corporation. While IPOs are effective at raising capital, being listed on a stock exchange comes with heavy regulatory compliance and reporting requirements.
The term IPO only refers to the first public issuance of a company's shares. It assumes a company is big enough, successful enough, and has the required track record to raise capital in the public equity market. If a company later sells newly issued shares again to the market, it is called a seasoned equity offering. When a shareholder sells shares, it is called a secondary offering and the shareholder, not the company that originally issued the shares, retains the proceeds of the offering. These terms are often confused and only a company which issues shares can make a primary offering or IPO. Secondary offerings occur on the secondary market, where shareholders (not the issuing company) buy and sell shares from and to each other.(Read more at Wikinvest )