Form 13-D is commonly referred to as "beneficial ownership report," and is required when a person or a group of persons acquires beneficial ownership of more than 5% of the voting class of a company's equity securities; form 13-G is the abbreviated version of the form that is allowed under certain circumstances.
The information in forms 13-D and 13-G is extremely timely as it is required to be filed within ten days after the purchase, in contrast to 13-F quarterly filings by Institutions that are filed every three months. The information contained in 13-F filings, thereby, can as much as eighteen weeks old by the time it is disseminated to the public. Furthermore, by virtue of their 5% ownership in public companies, the information contained in the 13-D and 13-G filings indicates only high confidence or high conviction moves by institutions and insiders, and hence can be interpreted to be of greater relevance to the investment community than the 13-F quarterly filings. Furthermore, 13-D and 13-G filings often are a precursor to hostile takeover, company breakups and other "change of control" events, and often they will include a letter to management explaining the reason for their taking a large stake in the company.