Java Content Repository
The Java Content Repository (JCR) standard, JSR 283 , specifies a vendor-independent and implementation-independent way to access content bi-directionally on a granular level within a content repository.
Specification lead is held by Adobe Research (Switzerland) AG.
The JCR API 2.0 package, javax.jcr.* is used for the direct access and manipulation of repository content.
Experience Server (CRX) and Jackrabbit
The Experience Server provides the Experience Services which AEM is built on, and which can be leveraged to build custom applications, and it embeds the Content Repository based on Jackrabbit.
Apache Jackrabbit is an open source, fully conforming, implementation of the JCR API 2.0.