Wikipedia explains the SEC’s S-1 form as:
The S-1 contains the basic business and financial information on an issuer with respect to a specific securities offering. Investors may use the prospectus to consider the merits of an offering and make educated investment decisions.
The full wikipedia article is here:
According to Investopedia:
Form S-1 requires companies to provide information on the planned use of capital proceeds, detail the current business model and competition, as well provide a a brief prospectus of the planned security itself, offering price methodology, and any dilution that will occur to other listed securities.
The full Investopedia description is available here:
In case you’re ready to file, you can see a blank copy of the form here:
And here is Facebook’s completed S-1 form: