An initial public offering, or IPO, refers to when a company first sells its shares to the public. For more information about IPOs generally, see our Investor Bulletin on IPOs. You can also find fast answers on why investors have difficulty getting shares in an IPO, the pricing differences between the IPO and secondary market trading, a brokerage firm's IPO eligibility requirements, and lockup agreements.

http://www.sec.gov/answers/ipo.htm