Apache Karaf can work on top of any of the two most used OSGi frameworks: Apache Felix or Equinox (OSGi), providing additional features.
Karaf Container is a modern and polymorphic container. It's a lightweight, powerful, and enterprise ready container powered by OSGi. By polymorphic, it means that Karaf can host any kind of applications : OSGi, Spring, WAR, and much more. Karaf can be used as a standalone container, or in a bootstrap way using Karaf Boot.
Karaf Cellar is a clustering solution for Karaf. It allows you to manage multiple instances, with synchronization between the instances.
Karaf Cave is an implementation of OSGi Repository specification. It can be used by the Karaf Features Resolver to provide resources, dealing with the requirements and capabilities of artifacts.
Decanter provides ready to use monitoring solution (including alerting key feature) or BAM (Business Activities Monitoring). It's also completely extensible and customizable.