MIDI controllers

 

They come in all sorts of sizes from tiny 2 octave things right up to weighted piano key 88 note monsters. They vary a lot in their capabilities the cheaper ones won't have aftertouch response, most of them respond to velocity.

 

They all require you to connect to a sound source of some kind - either a real synth module or a computer running virtual instruments.

 

Some allow you to zone or split the keyboard so it can send MIDI say from the top 3 octaves to a module or VSTi running a Moog sound, the central 3 octaves to a module or synth running a melotron sample and the bottom octave and a bit to something emulating a Taurus bass. (Just one example). Another might be to layer sounds from different synths and maybe define which one gets played based on velocity (Play soft, get a string sound from synth A, play hard, get orchestral brass from Synth B)

 

Many of them these days also have banks of knobs and sliders allowing real time control of parameters on your synths or VSTis. Some allow you to set up and recall scenes and presets, making complex patch changes live a bit easier.

 

Several of the bigger synth workstations can double quite comfortably as a controller for other synths or modules.

 

They run in price from sub 100 UKP to maybe 500+ for a big weighted key 88 note controller.

 

Common makes are Fatar, Evolution, Studiologic, M-Audio.  Roland's AX30 is the one you can wear slung like a guitar and strut like your guitarist buddies

 

Hope this is helpful

 

But what will you control with it?