In C is a minimalist piece by Terry Riley. It consists of a page of musical fragments and some instructions to performers. As the piece is played, each musician loops round a phrase (at an agreed tempo) for a period they choose and then moves onto the next. The result is that the piece moves forward coherently but the exact combination of textures and the overall feel will vary each time.
Words on their own don’t really do it justice though – David Bruce explains it well in the video below and there are plenty of versions of the piece available online.