Introduction to Pro Tools : Pro Tools Concepts : Pro Tools Sessions : Playlist

Playlist
A playlist is a sequence of clips arranged on an audio, MIDI, or video track. Tracks have edit playlists and automation playlists.
 
On audio tracks, an edit playlist tells the hard disk which audio clips to play in what order. For example, you can have separate audio clips for a song introduction, the first verse, the first chorus, and so on. You can also use the same audio clip to access the same piece of audio multiple times at different locations and not use additional disk space. Different versions of the same original audio can be used in different places and have different effects applied. On MIDI and Instrument tracks, edit playlists can store multiple MIDI sequences (or performances) on a track.
A playlist can be made up of a single clip or many separate clips. It can be made up of similar elements (such as clips from several different takes of a solo), or dissimilar elements (such as several sound effects).
You can create any number of alternate edit playlists for a track. This lets you assemble different versions of performances or edits on a single track and choose between them from the Playlists menu on the track.
Each audio, Auxiliary Input, Instrument, Master Fader, and VCA track also has a single set of automation playlists. Automation playlists can include volume, pan, mute, and each automation-enabled control for the insert and send assignments on that track.
MIDI controller data on Instrument and MIDI tracks is always included as part of the track playlist.