Low quality MP3s reduce full audio to 1/10th of the original size, while FLAC maintains all the information from the original recording, thus ensuring no loss in sound quality.
FLAC is the best possible way to download classical music for CD quality, however it is not natively supported by some media players (including iTunes and Windows Media Player). Certain applications are available that enable you to listen to FLAC files through iTunes. These include Fluke for Mac.
Alternatively, VLC and Songbird are players that can both be used to play Flac Files on Mac and PC.
High quality MP3 320 kbps offers near CD quality and can be played by almost all desktop players and portable devices such as iPads and iPhones.