A new feature introduced in Mac OS X 10.8 (Mountain Lion), PowerNap allows a Macintosh to continue performing certain tasks even through the computer has been put to sleep. For example, with PowerNap, a Mac can still receive e-mail, update software, sync calendars, and make backups with Time Capsule.
At this time, PowerNap is only supported on the newest MacBook Air and MacBook Pro Computers, since built-in flash storage is required (in other words, an SSD or solid-state drive).