MPICH2 is an implementation of the Message Passing Interface (MPI). The goals of MPICH2 are to provide an MPI implementation for important platforms, including clusters, SMPs and massively parallel processors. It also provides a tool for MPI implementation research and for developing new and better parallel programming environments. MPICH2 replaces MPICH1. [Source: MPICH2 Web page]
See also Message Passing Interface (MPI).