JCR is the acronym of the JSR 170: Content Repository for Java technology API, a standard interface for accessing content repositories. Jackrabbit is the reference implementation of that standard and the most popular Open-Souce implementation. It is full-featured and used in production environments. JCR-Explorer uses Jackrabbit for development and testing.
The JCR-Explorer is a web-based tool to administer a JSR-170-based content repository like Apache Jackrabbit. For example it allows to browse and edit content, query the repository or view repository information.
Due to its web-based nature all users having access to the server your repository is running on can also use JCR-Explorer to browse it.
Alternatively there are solutions available running in Eclipse (like JCR Browser, also on SourceForge) or using Java WebStart (like JCR Controller). The choice is on you!