Form 13F is a quarterly report filed, per United States Securities and Exchange Commission regulations, by "institutional investment managers" with control over $100M in assets to the SEC, listing all equity assets under management.
Form 13F provides position-level disclosure of all institutional investment managers with more than $100m in assets under management with relevant long US holdings. All US-listed equity securities (including ETFs) in the manager’s portfolio are included and detailed according to the number of shares, the ticker, the issuer name, etc. The full list of securities currently covered by form 13F includes more than 17,500 securities. Short positions are not required to be disclosed and are not reported. Form 13F covers institutional investment managers, which include Registered Investment Advisers (RIAs), banks, insurance companies, hedge funds, trust companies, pension funds, mutual funds, [clarify]
Form 13F is required to be filed within 45 days of the end of a calendar quarter, or if that day falls on a Saturday, Sunday or holiday, the deadline is the next business day. So, for example, for positions held as of the end of December, Form 13F must be filed by February 14 (if it is not a Saturday, Sunday, or holiday).
Both the Chartered Alternative Investment Analyst (CAIA) designation and the CFA Institute make special mention of 13F-based analysis for due diligence and monitoring of investment managers, as well as investment-idea-generation. CAIA includes discussion of  in its study materials, as well. Absent access to privileged access to manager holdings, Form 13F provides a baseline level of data with which to analyze investment manager exposures, performance attribution, and associated risks.