S-1's are the forms used in an IPO to notify the SEC of the upcoming public offering of the company. This is a pre-requisite to being listed on anyexchange (and by extension to completing the IPO).
The S-1 form contains the relevant information about the company and offering it is making. This is usually less informative to investors (but also less biased) than the corresponding prospectus that is used to create investment in the newly-public company.
The financial advisor handling the IPO will go to great lengths to make the company seem as attractive an investment as possible whereas the S-1 is just a statement of facts to the SEC, so investors are advised to examine both before investing.