An Initial Public Offering (IPO) is the first listing of a security on a public exchange. Companies making an IPO need to comply with the registration requirements of the 1933 Act (pdf) as well as the public reporting requirements of the 1934 Act (pdf).
To complete an IPO a company must engage banks as underwriters who can value the company and price securities and ultimately distribute them, and the company must market the securities to potential investors. To get approval from the SEC to move forward with an IPO, a company must file an S-1 Registration, which includes a prospectus on the company and other detailed disclosures.
Once the S-1 is filed, the SEC makes comments that require revisions to the form, and the back and forth typically takes ten to twelve weeks before the securities can be publicly listed.