When you say MIDI keyboard you're talking about a variety of electric or digital keyboards out there. Unlike before where only special keyboards were able to do MIDI, nowadays almost all modern electric keyboards are MIDI compatible. You can't miss them - they're the ones with the MIDI In & Out slots in the back.
There are basically two types of electronic keyboards - the controller and the piano types
1) The controller is typically what many would associate with the term MIDI keyboard. While it has keys you won't hear any sound when you press them. You need to have a computer running a digital audio workstation (DAW) software to convert the MIDI impulses to sound.
Because they don't have any speakers controllers are generally smaller and cheaper than their regular piano type counterparts. This makes them versatile and portable and can fit in almost any space you can think of.