Perhaps the clearest, most objective explanation comes from the tech gurus at Ars Technica, and can be read here: http://arstechnica.com/apple/2012/04/does-mastered-for-itunes-matter-to-music-ars-puts-it-to-the-test/ Basically in a nutshell, mastering a release for iTunes involves starting with the highest quality source possible (24 bits and 96 kHz) and then processing it specifically with the iTunes AAC format in mind, so that the end product has a fuller, richer sound and a broader dynamic range.
Obviously this is particularly important for classical music, which often has a very nuanced sound palate and a much wider differences between the softest and loudest portions (compared to the average pop song or dance track). So we’ve had our experts at Abbey Road studios process more than 80 of our most important recent and historic releases in this new Mastered for iTunes format, and we think the results speak for themselves.
[video of the EMI Classics albums now Mastered for iTunes]
Enjoy! And please let us know what you think in the comments.