If MPI is Message Passing Interface, then what is MPICH ?

Question

Can anyone defines clearly what is MPICH ?
For what it is used ?
Its relation with MPI.
Why do we need MPICH ?

Answer 1

MPI is a standard interface definition. ie - it defines how to program to it, but doesn't provide an implementation.

MPICH is a specific implementation that conforms to that interface, and is portable to a huge number of platforms. OpenMPI (not to be confused with OpenMP) is another implementation, as is LAM, and many vendors have their own implementations tuned to their platforms. If you write your program to conform to the MPI standard, you can link to any conforming implementation.

MPICH was one of a handful of reference implementations that became widely available in the mid-90's.

MPICH is to MPI as GNU libc is to the C standard library.