Ok, here goes...
a workstation contains lots of onboard sounds which attempt to replicate the sounds from *real* musical instruments. If you select, say, an accordion sound on a workstation, then when you hit a key on the keyboard you'll be triggering an audio sample taken from a real accordion.
Workstations also usually have onboard sequencers and onboard floppy disc drives for recording and playing music which has been saved in midi format. The downside to workstations is that there's not much room for tweaking the sounds and you're pretty much stuck with the factory presets.
A workstation would be a better choice for someone who was trying to compose (for example) some classical music.
Synths on the other hand are more geared towards people who want to create their own unique (freaky!) sounds. You can normally tell the difference between a synth and a workstation at a glance, because synths usually have lots of knobs and/or sliders for tweaking the sounds.
Also, the way in which synths produce sounds is different from with a workstation. Instead of using sounds which have been sampled from real instruments, synthesizers use difference methods for creating sounds such as producing a sawtooth wav and adding filter sweeps and so on. Although both synths and workstations have presets which attempt to replicate the sounds of real instruments... it's generally agreed that workstations sound more realistic because of their totally different method of sound production.
Both (modern) synths and workstations can be programmed to respond to midi data.
The only other thing I can think of that's worth mentioning is that the difference between synths and workstations seems to be becoming more blurred every year. Powerful instruments like the Korg Triton for example are often described as "workstation synthesizers" because they can pretty much do all the things a workstation can do, as well as doing all the things a synthesizer can do.
In short... if you wanna compose traditional sounding music (pianos, guitars, harps, flutes etc), then you want to lean towards the side of a workstation.
If however, you wanna produce Madonnas next album, then you should go for a synth.
-Equalizer
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David