Playlist - A playlist is a selection of custom-ordered songs created in iTunes or other digital audio programs.
For instance, if you wanted to just hear every version of "The House of the Rising Sun" that you have in your iTunes library, no matter what artist performed the song, you could create a playlist for this and then listen to those versions in order. You could also create a playlist of Christmas songs, or love songs, or songs about Texas.
Playlists can be created from any audio content in iTunes and given whatever order of playback you want. They can also be added to iPods, though they have to be created in iTunes and then synched.
A playlist is roughly the digital equivalent of a mixtape.
More at Wikipedia.