What is the difference between service and channel?

Channel is a path that can be physical or virtual. Service is a set of transparant (not visable) jobs that give a preset outcome. Example: Telephone service. You pick up the phone and dial a number. The phone on the other end will ring.

Example: Ask a waiter for coffee and donut. The waiter will do that task for you. You however will not know all the tasks he will have to perform.