For the uninitiated, a Controller Keyboard (also called a MIDI Keyboard or Master Keyboard) is a set of keys and controls that allow you to send MIDI signals.
Those signals tell your software or a rack/module to play notes, change sounds and much more.
This makes a controller a great feature in studio and software-based situations, including live.
Mega Music have all the good ones, brands like AKAI, Native Instruments, M-Audio, Korg and Roland.
Available to order here online or to check out for real at a Mega Music store.