Sakai's content has historically (through the 2.5 release) been managed by the Content Hosting Service, or CHS. The future direction of content hosting in Sakai now angles away from that approach, toward using the Java Content Repository API (currently JSR-170) and its implementations.
The long term vision is that a JCR-backed Content Service will replace Content Hosting Service and all tools manipulating content will use the new service. The new service (JCR Service) will be a minimal shim on top of a JSR170/JSR283 interface to a compliant repository. Sakai will ship with Jackrabbit as the default repository implementation accessed through the new service.
JCR describes the class of standard/specification of which JSR-170 is the first version and JSR-283 the second. By this terminology both JackRabbit and Xythos are implementations of the JCR API and are colloquially referred to also as "JCRs" (as in Java Content Repositories) as well as implementations of the JCR API.