The octave buttons are used to shift the keyboard up or down in one octave increments. This extends the range of the keyboard in either direction, which lets you play notes that lie outside the number of keys available.
A feature on some MIDI devices that allows to adjust tempo in real time based on an external input or by “tapping” a button on the device itself. It is used to allow musicians the flexibility of playing at any desired tempo based on their musical input.
Transpose buttons are used for moving a collection of notes (pitches or pitch classes) up or down in pitch by a constant interval, usually in semitone increments.
The more buttons the device has, the more complex the input can be.
The arpeggiator is the main tool used by computer musicians to generate looped material. An arpeggiator is simply a generator of signals that controls the tempo, pitch, velocity, gate, and other parameters of the sound.
The transport controls are used to control playback, set the position of the playhead, start recording, activate the cycle region, or to move to the project start or end.