Form 8-K is a very broad form used to notify investors of any material event that is important to shareholders or the United States Securities and Exchange Commission. This is one of the most common types of forms filed with the SEC. After a significant event like bankruptcy or departure of a CEO, a public company generally must file a Current Report on Form 8-K within four business days to provide an update to previously filed quarterly reports on Form 10-Q and/or Annual Reports on Form 10-K. Form 8-K is required to be filed by public companies with the SEC pursuant to the Securities Exchange Act of 1934, as amended. For a list of events that would trigger a Form 8-K to be filed, see the Official SEC Form 8-K Summary, reproduced below.
When Form 8-K is required 
Form 8-K is used to notify investors of a current event. These types of events include:
- Material definitive agreements not made in the ordinary course of business
- Bankruptcies or receiverships
- Director is elected
- Director departs
- If a director departs because of a disagreement with the company on any matter relating to the registrant’s operations, policies, or practices then an 8-K must be filed to disclose a brief description of the circumstances representing the disagreement.
- Asset movement: acquisition or sale
- Result of operations and financial condition
- Material Direct Financial obligations (bonds, debentures)
- Triggering events that accelerate material obligations (defaults on a loan)
- Exit or disposal plans
- shutting down a plant
- material change in services or outlets
- Material impairments
- Delisting or transfer exchange notices
- Unregistered equity sales
- Modifications to shareholder rights
- Change in accountant - and good idea to explain why
- SEC investigations and internal reviews
- Financial non-reliance notices
- Changes in control of the company
- Someone takes a large equity position (more than 15%); shareholder also needs to file with SEC as to intent
- Someone takes a 51% position
- Changes in executive management
- Officer leaves
- Officer is fired
- Officer is hired
- Departure or appointment of company officers
- Amendments to company Governance Policies
- Code of Ethics
- Board Committee Governance Policies
- Trading suspension
- Change in credit
- Change in company status
- Other events
- Financial exhibits
Investors should always read any 8-K filings that are made by companies in which they are invested. These reports are both material and relevant to the company, and often contain information that will affect the share price.
Reading Form 8K 
Typically an 8-K filing will only have two major parts. They are:
- The name and description of the event - this contains all the information that the company considers relevant to shareholders and the SEC. It is important to read this information, as it has been deemed "material" by the company.
- Any exhibits that are relevant - these exhibits may include financial statements, press releases, data tables, or other information that is referenced in the description of the event.
- Hewlett-Packard Company, SEC Administrative Proceeding 3-12643 (Securities and Exchange Commission 2007-05-23).