It should be noted that Fedora is in the process of reducing its dependency on Modeshape as part of an ongoing effort to completely remove it from the Fedora codebase. Keeping that in mind, it is advisable to avoid creating new dependencies on Modeshape or even the JCR API unless it is absolutely necessary.
ModeShape is a JCR implementation with a slew of additional services and functionality, including a REST/HTTP API, additional query languages (including full-text search), clustering and federation, and automatic disassembly of ingested content into repository nodes.
From the ModeShape website:
ModeShape is a distributed, hierarchical, transactional, and consistent data store with support for queries, full-text search, events, versioning, references, and flexible and dynamic schemas. It is very fast, highly available, extremely scalable, and it is 100% open source and written in Java. Clients use the (JSR-283) standard Java API for content repositories (aka, JCR) or ModeShape's REST API, and can query content through JDBC and SQL.
Please see the ModeShape project documentation for more information.