MIDI Controllers are typically used as an interface to input and trigger sound in computer software, such as DAW (Digital Audio Workstation). Most are compatible with iOS, Mac & PC platforms.
They come various formats and sizes, generally, as a small traditional piano keyboard or as a geometric unit similar to an electronic drumpad. Unlike a digital keyboard, the sounds generated are provided via the computer software and the computer itself.
Here at Yamaha Music London we sell a variety of both types from reputable manufacturers such as the Line 6 Mobile Keys range and Novation's Launchpad series.