The MPI standard has been defined in ``MPI: A Message-Passing Interface Standard''. That document was produced by the Message Passing Interface Forum, and is available from the World Wide Web server at the University of Tennessee at Knoxville, http://netlib.cs.utk.edu/. When you will have made the connection, go to the Netlib Repository, and then select mpi. The file to transfer is mpi/mpi-report.ps, which contains the MPI Standard specification as of May 5, 1994. The report is 228 pages long. When printing the report you may try to put 4 pages on one sheet of paper using the multi.ps package for PostScript from Ross Cartlidge, rossc@extro.ucc.su.oz.au.
This document is a must, if you are really serious about using MPI. It is quite readable and contains numerous examples and explanations.
MPI Standard document is being converted to html and it is already available from www.mcs.anl.gov. If you press here, the magic of Mosaic will take you all the way from Canberra in Australia (or from wherever you read this document now) to Argonne near Chicago in the US, and you will be able to view the MPI Standard document on line.