Outboard Gear

Samplers are the high end of synthesizing.  They are expensive, hard to learn to use, but are the best at generating custom built sounds or recording realistic sounds.  Basically they record a sound directly and then edit it to give it a sound envelope such that when it is played back it produces a realistic version.  The sound is "looped" to obtain the sustain portion.  So a lot of clever manipulation of the sound has to be done.  Also you can scale the sound up or down in pitch for a short range without it changing the timbre too much.  Then sample the sound again at a new pitch and build a library of such sounds that covers an useful keyboard range.

FM Synthesis

Just after the analog craze of ganging oscillators together to get full sound such as from the Moog synth or the Oberhiem and Arp companies that built wonderful full sounding machines, Yamaha built a keyboard using FM synthesis or the combining of sine waves and few other waveform types to generate bell like metallic and sometimes down right weird sounds.  The DX-7 was born.  It was portable 76 key machine built in a steel cabinet that was very road worthy.  They were everywhere for a while.  The sound quality was limited, and was replaced by waveform sampling as a basis for the tones generated, and all the manufactures got on board with this "new wave", and flooded the market with synthesizers.  Basically, all the newer boxes are great sounding, and therefore, are a personal choice.  Piano is still a beast to copy electronically.  This is mainly do to all the harmonic overtones generate passively by the body of strings within a  piano that vibrate sympathetically whenever a hammer hits an individual string set.  If you know pianos, then you know that most of the notes are sounded from a felt covered hammer hitting either one, two or three strings tuned to the same pitch.  Most of the keyboard range is striking three strings tuned to the same pitch.  This provides volume balance across the keyboard range, fullness, and gobs of harmonic content.  Each combination of notes creates a new set of overtones, which are very difficult to duplicate electronically.  Sampled real sounds of each note would be a good approach, but the mass of memory to record it becomes unwieldy and you would still have the problem of overtone sets for any given set of strings being played. That's getting close to infinity.  Trying to record all these possible combinations of chords to capture the different harmonic patterns would be impossible.  Well, this century anyway.

Reverbs and Effect Boxes

Here is a news flash.  Electronic sounds have no ambience.  No life in space.  No Mojo.  Electronic sound is a flat, two-dimensional sound.  Real spaces like in a music hall or a room has reflections of the sounds generated that bounce off the walls and regenerate the sound modified by the surface as it arrives at the listener at different times to produce the overall effect of hearing quality sound.  That's hard to duplicate in the electronic arena.  Lexicon is a common brand that has done it well and if you can afford about $2,000 to get a PCM-70 you can get a decent sound out.  The lesser boxes are well, lesser.  But you need this effect of ambience.  Most units have it built in now days.  It's okay.  There are many other types of "special" effects.  Some units clean up inter-modulation noise created when signals are added together.  Mixing signals generates shadow waveforms that need to be cleaned out that are at frequencies of double and one half the combination of the original inputs.  Given a bunch of inputs, this can get hairy.  These harmonics are very weak in volume and are for the most part, not of much concern.  Only in million dollar studio quality set ups.  They care.  Of course, guitar players are used to a myriad of effects boxes that provide amazing sound modification.  Most all of this is unnecessary except for reverb in the basic recording end of things.

Workstations - Studios in a box

This is the all-in-one solution, and is not a bad one.  You can't modify what you get, but if the package is a good one, you don't need to go further.  Many have add on plug-ins that provide more sound sources, but the zillion onboard patches are probably more than you want or need anyway.  Many have a disk to record sequences to.  Some come with a CD burner built in.

Keyboard Workstations

These multi-taskers not only have a nice keyboard, but they serve as a MIDI controller, that broadcasts to other equipment connected via MIDI cables to make program changes, patch changes, or any other trick the user wants to set up.  One live performance advantage is programming a key that when pressed, triggers a music sequence or arpeggio or a foot pedal that starts a drum machine.  Split keyboards offer a way to play bass in the left hand and lead in the right hand, not to mention using foot pedals to activate drum fill patterns at exactly the right time.  Along with the performance features, the workstation keyboard has sequencing software built in.  This allows the workstation to be a full function production center.  Multi-tumbrel, polyphonic, record and playback monsters. Get your checkbook out.

Combos

The next unit that is marketed to dazzle the consumer is a stand alone box that usually has a full rhythm machine, synthesizer sounds, and sequencer built in.  Very useful for bands that need a drummer, but can't find a human one.  Just enough horsepower to sell well, but not overwhelming. 

Studio in a Box or Digital Analog Workstation (DAW)