A registration statement is a document filed with the SEC by companies to register securities for public sale, providing investors with crucial information about the company's business, financials, and risks. The type of form required, such as the comprehensive Form S-1 for initial public offerings (IPOs), depends on the specific type of offering and the company's status. Other types include Form S-3 for shelf registrations, Form S-4 for mergers, and forms like F-1 for foreign companies.