My understanding, based on working with the product since January 2011 - The original functionality was all based on agents, the agent was installed to do one thing, and it could only do one thing. It connects to the manager service via SOAP, and requires installing a dedicated agent for a particular type of endpoint. The DEM communicates with the web server via REST, it does not maintain a personality like an agent, but pulls down the required workflows at runtime. This allows you to update a given workflow one time centrally, and all the DEM's will run that new version next time they execute it.
There was an architectural decision after the DEM was introduced, that new functionality would be written into the DEM type platform and originally DynamicOps was talking about moving the agent functionality into the DEMs (never happened). So you will see things like Amazon EC2, physical provisioning, calling vRO, etc. are executed by the DEM Workers, but the original core functionality of the product (vSphere, Hyper-V, etc) have associated agents that execute that work.
