The Parallel ROOT Facility, PROOF, is an extension of ROOT allowing transparent analysis of large sets of ROOT files in parallel on remote computer clusters or multi-core computers. PROOF is designed to work on any ROOT object, but the usual case of massive data analysis mostly involves accessing ROOT trees.
PROOF tries to show as little difference as possible with respect to a normal ROOT session, while providing good scalability and insensitivity to changes of the remote environment (for example, network or node problems). When pure interactivity is not possible, the user can still operate in "interactive batch" mode: start very long running queries, disconnect and reconnect later on to monitor the status or retrieve the result.
More information about PROOF can be found in the PROOF book.