A prospectus means any invitation issued to the public inviting it to deposit money with the company or to take shares are debentures of the company such invitation may be in the form of a document or a notice for a circular or advertisement etc.
the term prospectus includes two types of prospectus.One is Red Herring Prospectus And the another is shelf prospectus
The term shelf prospectus means a prospectus which are issued by a company for subscription in one or more issues over a certain period without the issue of a further prospectus.
Sometimes, it happens, that securities are issued in stages spread over a period of time, particularly in respect of infrastructure projects where issue size is large as huge funds have to be collected .In such case, filing of prospectus each time will be very expensive. So , a provision was made namely shelf prospectus in the section 31 of Companies Act 2013 .The advantage is that at each stage of offer of securities during validity of shelf prospectus filing of prospectus is not required ,which reduces the cost of the company