iCloud: iTunes Match
How it works
iTunes determines which songs in your collection are available in the iTunes Store. Any music with a match is automatically added to your iCloud library for you to listen to anytime, on any device. All the music iTunes matches plays back at 256-Kbps iTunes Plus quality—even if your original copy was of lower quality.
For more information, open iTunes and choose iTunes Help from the Help menu, or visit the iCloud website.