PowerNap is the name and it's certainly an interesting addition to the OS but it will only be making its way select devices such as the second-generation MacBook Air and Next Generation MacBook Pro. So what does it do?
PowerNap will auto-update your system while it is asleep. App updates, System updates and even backups can now all be done in what could be considered, downtime for yourself. As noted on stage, it's power efficient so it shouldn't draw too much power from the battery in order to complete the tasks at hand.