Analogue synths were voltage controlled that is, it was actual volts and amps that created the sounds, oscillators, ADSR modules, ring modulators, hence they were prone to go out of tune in temperature extremes.
Digital synths have the sounds pre programmed into the chips & IC's.
The classic british synthesiser was the VCS3 developed my EMS..the BBC had a huge synthesiser in their Radiophonic workshop.
King Crimson, Roxy Music, Hawkwind were some of the 60's/70's prog rock bands that used them. A unique sound in themselves..just like old fuzz boxes and tape reverb units.