A software-based musical sequencer is a program that composes several sequences of digital audio together from a number of operations. In the case of a MIDI sequencer, audio is stored in the form of a series of events, meaning that it does not record the actual audio but the sequence of actions taken on each time instance — which note was pressed at what time and for how long.

Hardware-based musical sequencers are generally lightweight and portable. Modern sequencers are frequently in the form of a keyboard, and can help in controlling and composing music out of virtual instruments. They can help musicians by replacing individual synthesizers.