Form 13F is shorthand for the quarterly report filed, per SEC regulations, by "institutional investment managers" to the SEC and containing all equity assets under management of at least $100 million in value.
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, among natural persons or entities with investment discretion over its own account or another's.
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. David Larrabee, the director of Member and Corporate Products at the CFA Institute, regularly publishes 13F-based holdings analyses of institutional investors. 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.
More sophisticated investors can apply the Brinson Fachler performance attribution methodology to the time-series of 13F disclosures to assess security selection skill, sector picking skill, and position sizing skill. Calculating StockAlpha, for example, provides a metric on value derived from a manager's stock picking.