Definition of Terms
The following terms are used frequently in literature. Despite of the statement in
the heading, there are no hard definitions, but the following should reflect common
usage:
-  Model
        A model is a description of the device which behaves just like the device does.
        Generally the behaviour can not be controlled by any other way then applying
        stimulus to it's input pins. RAMs are usually described in this way. 
 -  Transactor
        Transactors have additional control mechanisms: they can read control data from
        a file or contain test specific code. They control which test is run in the
        testbench in which they are instantiated. 
 -  Bus Functional Model (BFM)
        A BFM is a model of a device as it appears on a bus. E.g. a processor model
        coded in this fashion would only contain read and write processes, and none
        of the internal processing (no ALU, registers etc).