Major developments that investors should know about are described in the 10-K or 10-Q, but if those developments don't make the two filings in time, they are presented in the 8-K. This unscheduled document addresses specific events and provides further detail and exhibits, such as data tables and press releases.
Events that lead to the filing of the 8-K include a bankruptcy or receivership, material impairments, completion of acquisition or disposition of assets, departures or appointments of executives and other events of importance to the investor.