Apple's trying to differentiate iTunes from its competitors with a new "Mastered for iTunes" section that's filled with high-fidelity, ear-pleasing music. These tracks are processed using a set of guidelines and tools that'll maintain as much of the sound quality of the original, uncompressed file as possible.
As Ars Technica points out, most modern music tracks are recorded using 24-bit samples at 96 kHz or 192 kHz, depending on the processing power and storage space of the system. iTunes files are then compressed to a 256 kbps AAC "iTunes Plus" format, which loses 97 percent of the data contained in the original, uncompressed recording.
Apple's new "Mastered for iTunes" tries to minimize this lost data by downsampling the original, high-quality music file to 44.1 kHz using a 32-bit floating-point intermediary file. This file is then converted to AAC. As Apple writes in its Mastered for iTunes guide, this process uses "every bit of resolution available, preserving all the dynamic range of the 24-bit source file." For end users, this means your Master for iTunes tracks will sound delightful to even the best-trained ears.
For more details on the technology behind these new Mastered for iTunes tracks, you can visit Apple's new Mastered for iTunes website. There's also an excellent article from Ars Technica on the subject.