It's generally assumed that computer games are playful, yet highly structured experiences. That's certainly fair comment -- there are set rules, mission structures, or goals to achieve. Games are won or lost, and rules can rarely be changed. Stories and events are often highly scripted and pre-determined, with at least partly set pacing.
Googling unstructured play brings up a lot of articles about the importance of a freeform style of play for kids, and the need to have time away from structured play like video games. I don't disagree, but of course it's not really that simple. Structured vs. unstructured play isn't a true binary option, it's just one way of looking at a wide spectrum of play. Still, I'll run with the distinction for now, because I think it leads some interesting places.