MIDI, or Musical Instrument Digital Interface, is digital information channeled through a software instrument to produce sound. Pictured above are three MIDI tracks, and MIDI regions in green. Under those regions in larger green bars, the MIDI editor in GarageBand allows for fine-tuned editing of the selected MIDI region.
Watch the video below for more information.
Drummer is an Apple feature present in GarageBand and Logic Pro X. Create a Drummer Track in the Track menu to quickly create drum sequences. While not true MIDI, Drummer relies on similar principles with a separate interface and controls.
For more details on Drummer, watch the video below.