They're any object. Specifically, objects in the kitchen or plumbing category break far more often, despite being used as much as the electronics. Not sure if it's related, but Sims keep cooking without eating the food they already made (probably not, but maybe?).
So far (since I don't like townies), I've only tested with other Sims in my household. To get Sims to fix things, I physically have to click on their tab, click on the object, and say "Repair" before returning back to my Sim. Since I'm playing the Asylum challenge, only one of my Sims has ever fixed anything.
