carpediem . A bit complex to explain ATC in 10 lines .It takes a good year of training for a controller to get throught the stuff.
Short version :
ATM is Air traffic Management , it comprises Air traffic Flow management (ATFM) and Air traffic services (ATS) which itself contains Air traffic control ATC, plus the Weather services (MET) and Search and rescue (SAR).
ACAS is ICAO generic name for TCAS which is a brand name, it is airborne collision Avoiding system, An airborne system providing resolutions advisories (RAs) to pilots aimed at preventing a collision.
STCA is Short term conflict alert , a ground based ( i.e for ATC) system warning controllers when aircraft are on collision course and are about to loose separation ( normally 5NM and 1000 ft )