MPI Ruby is a Ruby binding of MPI. The primary goal in making this binding was to make the power of MPI available to Ruby users in a way that fits into the language's object oriented model. In order to do this, the buffer and datatype management necessary in the C, C++, and Fortran bindings have been removed. What this means is that MPI Ruby allows you to treat objects as messages.
MPI Ruby also aims to be a complete binding to MPI in that it offers access to nearly all functionality of MPI. While there is not a one-to-one correspondence to functions and constants in the Ruby and C/C++/Fortran bindings, all of the communication and topology features are available. There are fewer methods in the Ruby binding than there are functions in the C/C++/Fortran bindings, but this is mainly due to the fact that the programmer no longer needs to deal with buffers and datatypes.