Apple Lossless refers to the Apple Lossless Audio Codec, or ALAC. It was developed by Apple Inc. (then Apple Computer Inc.) for lossless compression of audio data. In contrast to MP3, OGG Vorbis, WMA and other lossy audio compression formats, lossless compression makes it possible to compress the audio data without losing any quality. The original track can be rebuilt from the lossless data. Other lossless audio compression formats include the Free Lossless Audio Codec (FLAC) and Monkey's Audio (APE).
Apple Lossless is currently supported by iTunes v.4.5 and up, after being introduced into QuickTime v6.5.1. All iPod players currently on the market support the Apple Lossless format, which is quick to decode and suitable for portable players. Apple Lossless data is stored in an MP4 container and uses the .m4a file extension. Currently, Apple Lossless is not subject to any Digital Rights Management (DRM) content protection system, but it is thought because of its use of the MP4 container, DRM could be easily added to the audio format.
Tests show that Apple Lossless compresses the original audio data to about 50 percent of its original size, as Apple claims. The level of compression depends on the data itself.