iTunes Match - $24.99 per year : Apple's cloud-based music service

・Automatically download new music purchases to all your devices
・Can download music you’ve previously purchased to all your devices
・Plays back from iCloud at 256-Kbps AAC DRM-free quality
・Limit 25,000 songs. (iTunes purchases do not count against limit.)
・Access to some services is limited to 10 devices

How to setup iCloud [Apple]

Memo: iTunesから購入していない曲も、自分のiTunesに取り込んでいる曲は、対応機器から自由に聞くことができる。