A MIDI keyboard will translate your physical motions interacting with the keyboard into MIDI commands that consist of the note/timing (what note/when you hit it), velocity (how hard you hit it)
and when you let go of the key (note length)
Some differences in keyboards to be aware of are the feel of the keyboard, the width of the keys, # of keys, other cool accessories like foot pedals and pitch wheels. Some have sequencers and/or synthesizers built in but these are completely unnecessary with a PC, a synth module and MIDI Software. Another cutesey add-on is the AD-LIB system.