A MIDI keyboard is a piano-style keyboard used for sending MIDI data to a computer, or other MIDI devices. MIDI keyboards are a type of MIDI controller. You can think of a MIDI keyboard as a dummy keyboard. They don't produce any sounds themselves but rather, via MIDI data, control sounds in other devices, such as a synth on your computer or an external hardware synth.
MIDI keyboards come in all shapes and sizes. In addition to piano keys, some have additional features such as drum pads and controller knobs. Drum pads are useful for playing in drum beats if you have a drum plugin such as Fabla. Controller knobs can be useful too if you want to control specific parameters of a plugin, eg. a synths volume control or it's filter cutoff.
The Akai MPK Mini is a compact, two octave keyboard that includes 8 drum pads and 8 controller knobs
MIDI keyboards come in two varieties, one's that you can connect up to any computer via a USB cable, and one's that use the traditional MIDI DIN connectors. If your keyboard only has MIDI connectors, you will need a soundcard with MIDI inputs to be able to connect your keyboard up to your computer.
A USB MIDI keyboard requires no additonal hardware, although you will only be able to use it with a computer, whereas a keyboard with MIDI connectors can be connected up to other MIDI equipment. Some keyboards come with either one connection type or another, other's come with both.