To me a synthesizer keyboard is a keyboard that can manipulate or combine waves or tones to generate or alter a sound. This can be done starting with a straight tone or starting with a "sampled" tone to mold it into the desire result. There are big differences in the capabilities of different synthesizers but they have at least some capability to change the sound, not just effects. There is a combination of sampled sounds and synthesized sounds on a keyboard. Sampled sounds require vast amounts of memory. To reduce the memory required use a few sampled notes on the scale with spaces in between. The notes in between the sampled notes are synthesized to create those notes. Kurzweil (one of the synths I have) has a "sampled" piano that takes very little memory. They also have a sampled piano that virtually every note was sampled. That piano requires 64 mg of memory. There are also pianos in between those 2 extremes, requiring different amounts of memory. This is determined by how many notes are sampled. Real instruments and all sorts of noises can be sampled.
Work stations are not that clearly defined. I see them as a keyboard that fits with your other equipment, computer, mixer or whatever to achieve the end product that you are wanting. Whether using samples, synthesized sounds or a combination is not the determining factor. "Workstation" is probably an advertising term.
[This message has been edited by bvan (edited 11-21-2002).]