An ALS file is an audio project, or "live set," file created by Ableton Live, a program used for creating electronic mixes. It saves the arrangement of clips on a multitrack timeline with mix settings and references to the clips, but not the clip files themselves. ALS files also support sampled instruments and looping.
Ableton Live is often used by DJs and live performers for saving and editing live dance mixes. The live set clips can be separated into different groups, which allow DJs to organize mixes by intros, choruses, and other sections as the music progresses.