Content Repository API for Java (JCR) is a specification for a Java platform API for accessing content repositories in a uniform manner. The content repositories are used in content management systems to keep the content data and also the metadata used in CMS such as versioning meta-data. The specification was developed under the Java Community Process as JSR-170 (Version 1) and as JSR-283 (Version 2). The main Java package is javax.jcr.
Version 3 aka JCR-333 of the JCR specification is currently being worked on and aims for a 2012 final release.
The liip team made presentation about Jackalope, JCR and PHP, check their slides
JCR and RDBMS/NoSQL