nowadays, in the home studio enviroment, you would probably be looking
for a machine that can connect to the computer and play samples or control
other parameters in it.
for that, the keyboard must be able to send MIDI commands.
some "organim" and [most] synths can send (& recieve) MIDI commands.
in that respect, a "mikledet shlita", or MIDI controler, or MIDI keyboard,
is simply an organ that doesn't produce sounds, like barak said,
but rather sends a numeric value with every operation made on it,
for an external machine to interprate.

adam