short: The bare C/C++ interactive interpretor CINT - used by ROOT
long:
The interpreter allows for fast prototyping of the macros since
it removes the time consuming compile/link cycle. It also provides a
good environment to learn C++. If more performance is needed the
interactively developed macros can be compiled using a C++ compiler.

For more information on CINT, please refer to 
     http://root.cern.ch/Cint 
