Just to clarify (i know the post is 2 years old) just in case other ppl are reading this thread for info:-

"General" keyboards are usually those with preset sounds and preset rhythms which are fixed. Almost always comes with built-in speakers. Good for home budding musicians /ppl who wants to play for fun and enjoy music at home. Its like a band in one easy package. The more expensive ones have more features (like touch sensitivity, after touch, song recording function etc.). Generally, most yamaha PSR series fall under this category.

Synthesizers are more "focused" keyboards. Usually no built in speakers (actually I've never seen one with it), and main difference is the sound can be edited (unlike the "general" ones where sound is fixed). Think of it like a sound creator. Usually fantastic sound quality. Some synths do have rhythm functions as well. If you know how to use one property, you can create amazing sounds. Keyboards that fall under this category are like the new King Korg, Roland Juno etc.

Workstations are basically mini studios. Comes with most or all of the features of a synth and a track recording function. Weighted keys as well for the 88 key versions (and sometimes on the 76 key ones like the Kronos). Examples include Motif series, Kronos series, Oasys. Can cost a small fortune.

Electric pianos at its most basic is just what the name suggest, just a piano with sounds produced electronically. Basic models have very few sounds (usually 2 acoustic piano / 2 digital piano / organ / strings / harpsichord ) More advance (and expensive) models are like scaled down workstations.

There are also other stuff like stage pianos (like the Roland RD series), arranger-workstation (which is like a "simplyfied" workstation), MIDI controller keyboard (which usually does not have any soundbank of its own and draws sound from a MIDI source like a laptop or a sound module).

The above is just a basic explaination. If want to go into details, I think can write a book LOL! Anyways, I stand corrected on all the above.

Happy keyboarding everyone!