A Musical Instrument Digital Interface (MIDI) keyboard is a musical instrument like a piano keyboard. The MIDI portion indicates that the instrument has a communication protocol built in that allows it to communicate with a computer or other MIDI-equipped instrument.
The MIDI interface is now so easy to implement that almost all keyboards sold today are some type of MIDI keyboard. This ranges from a simple 100 US dollar (USD) MIDI keyboard sold at the local department store to a 30,000 USD grand piano with a built-in controller. Every type can connect to any other type of musical instrument that sports a MIDI interface. The 30,000 USD instrument will sound much better than the 100 USD instrument, but both can be controlled by the computer or other instrument in the same way.
It is also possible to purchase a MIDI keyboard with no sound capabilities at all. This device requires the musician to have some other MIDI device that is capable of understanding the MIDI messages and generating the correct sounds. This other instrument may be a computer or a dedicated sound box with a number of different instruments programmed into it. This approach allows one company to concentrate on making the keyboard and another to specialize in recreating instruments electronically. It also allows the musician to upgrade the instrument without losing the familiarity of the keyboard.
When purchasing a MIDI keyboard, there are several things to consider. The two most important are your budget and your level of comfort with the keyboard itself. If you have an unlimited budget, the choice is easier; simply buy the best MIDI keyboard that feels good to you.
If your budget is more constrained, then concentrate on getting the keyboard with the best feel. You can always upgrade the sound quality at a later date. To upgrade the feel, you will need to get a new keyboard and learn to become comfortable with the feel all over again.
MIDI keyboards can have different amounts of keys. Classical musicians and purists will insist on all 88 keys being present. Keyboards with 76 keys are also popular. Keyboards with fewer keys are used when space is at a premium or for amateurs with a very limited budget.
MIDI keyboards come with a range of different feels. Some attempt to recreate the feel and action of a classic piano, while others have more of an electronic feel. Some are velocity sensitive, so that the faster the key is pressed, the louder the note sounds. Some are touch sensitive, so that the note sounds different depending on how hard the key is pressed. An inexpensive MIDI keyboard has neither effect.