The term MIDI is an abbreviation for Musical Instrument Digital Interface. MIDI keyboard pianos are actually part of setups used in music studios for producing music. Using a MIDI controller to play everything from piano pieces to bass lines, or even guitars or drums, gives a better feeling, a human touch. It’s far better than simply programming the notes with a mouse and a keyboard, although the end result is the same, a series of digital information resulting in a pleasant sound. A MIDI keyboard piano doesn't get you booked at Carnegie Hall. That comes from raw talent, skill, judgment, experience and psychological disposition.

For those that already know how to play a keyboard instrument, it makes the transition from playing to producing much easier, and it’s recommended that you certainly get a MIDI keyboard piano if you’re that kind of person. Although there are some models that will allow you to play them like a digital keyboard (the ones with 88 keys), they are still primarily made to help you in the studio when you’re producing music. It allows all the instruments and computers to communicate between them.

When producing music you need to get the information, the sound and everything else, from your musical setup to your DAW (digital audio workstation). The DAW can be an electronic device, or it can be a piece of software installed on a computer. To make the connection between the setup and the DAW, you need a MIDI interface. As such, it consists of the interface, the connectors that you can use to make a piece of hardware “communicate” to another, and the data language. 

A MIDI controller will be useless by itself, so it needs to work alongside other pieces of hardware, to form a setup. And for that it needs to connect to them, doing that through cables. So when discussing a MIDI keyboard, we need to take into account the whole ensemble. SOURCE: Musicality Realm.