Middleware is a technology(piece of software support) which is used in assisting applications built on N-tier architecture. MTS,MSMQ etc are middleware technologies as they assist u'r programs to perform certain functions with additional service provisions.
MTS is basically a Transaction monitor, but helps in scaliing your app by assisting in Thread,State and Memory management.
Read TedPattison's Prg COM with VB6 or Mary Kirtlands COM Prg with vb