Major release: A major, functional release with many new and improved features developed on an 18 month release cycle.
Continuous release (C): A functional release with some new and improved features along with new security updates, bug fixes for existing features, and previously released out-of-cycle patch updates.
Quarterly update (Q): A scheduled update that includes functional improvements, new security updates, and previously released out-of-cycle patch updates. For Reader, these updates are sometimes available as full installers.
Out-of-cycle patch (OOC): An update targeted at security fixes. These noncumulative patch files contain few functional updates with the intention to limit impact.
Note: Continuous releases, quarterly updates and out-of-cycle updates can provide security enhancements. Adobe recommends that all updates and patches be applied when available.