Atari STFM is the original ST, and FM stands for Frequency Modulation, i.e. you can stick it straight into a TV.
The Later STe means Enhanced.
STFm = 3 channel sound, 512 colours, little or no hardware.
STe = 4 channel sound, 4096 colours, extra hardware in the form of a barrel shifter (what we would call a blitter, only the Atari version is a joke! It can only scroll on 16pixel boundaries!!!)