ROOT/PROOF/XROOTD
First you should be familiar with ROOT,PROOF and XROOTD.

Now we will give some introduction for them.
ROOT
ROOT is a framework for data processing, born at CERN, at the heart of the research on high-energy physics. Every day, thousands of physicists use ROOT applications to analyze their data or to perform simulations.
* Processing data with ROOT (to learn about the interactive and batch use of ROOT)
* Analyzing data with ROOT (about histograms and trees; mathematical and statistical tools)
* Showing results with ROOT (explaining pads, 2D and 3D graphics, and the GUI editor)
* Simulation in ROOT (about the virtual Monte Carlo and the geometry packages)
* ROOT I/O (explaining how to use ROOT files and trees, and how to access data from d/b or other nodes)
You can get more detail information from:
http://root.cern.ch
PROOF
* Parallel ROOT Facility
* Interactive parallel analysis on a local cluster
* PROOF itself is not related to Grid
----Can be used in the Grid
----Can access Grid files
* The usage of PROOF is transparent
----The same code can be run locally and in a PROOF system (certain rules have to be followed)
* PROOF is part of ROOT
You can get more detail information from:
http://root.cern.ch/drupal/content/more-about-proof
XROOTD
XROOTD is a file system and Xrootd provides a Posix I/O interface.
You can get more detail information from:
http://xrootd.slac.stanford.edu