I'm assuming ATM means "Air Traffic Managment"
Which generally at least in this part of the world is called "ATS" or Air Traffic Services.
I think ATM/ATS are broad terms covering an entire network of systems. Centres/Towers/Flight Service etc.
While Air Traffic Control makes up a big portion of ATS/ATM, there are still many jobs/services that are provided to the aviation industry within the ATS umbrella which are not specifically Air Traffic Control.
Flight Planning/weather briefing just to name a couple of many.