RPC: Remote Procedure Call
MOM: Message Oriented Middleware

Both RPC and MOM are communication models for distributed systems.  Each has strengths and advantages. However, when you get into large heterogenous distributed systems, message passing is the way to achieve scalability.

I like this quote:

"In a large system you may be faced with either a multitude of clients or a menagerie of them; in either case you have to stop serializing objects and start exchanging documents."

- Joe Gregorio, 2007