What is ALAC
ALAC is short for Apple Lossless Audio Codec or simply Apple Lossless (also known as Apple Lossless Encoder, ALE) is a type of lossless audio compression created by Apple Inc for retaining all the original musical data. In contract to the lossy audio compression formats like MP3, WMA, OGG, etc, lossless compression provides the full fidelity of the original audio you compressed. Other lossless compression file formats include FLAC and APE. What's more, Apple Lossless can compress CD quality audio file to roughly half its original size.
Apple lossless files are stored within an MP4 container with the filename extension .m4a which is the same as the lossy AAC audio data (same container, different audio encoding). To distinguish a .m4a file encoded with ALAC or AAC, one way is to check their coding information and contents that display in iTunes. The big difference between ALAC and AAC is that ALAC can be identical to the original source.
The Apple Lossless Audio Codec project contains the sources for the ALAC encoder and decoder. Since April 28, 2004, this format has been supported by iTunes 4.5 and later, after being introduced into QuickTime v6.5.1. The Apple Lossless codec has gone open source, meaning that this lossless codec can now be freely used in both hardware and software. In addition, ALAC audio format can be obtained by transcoding from lossless WAV, APE, FLAC.
Apple Lossless supports 16 bit and 24 bit audio, multi channel audio and high frequency audio. If you'd like to make the ALAC conversion, see article: how to convert audio files to Apple Lossless using ALAC converter on Mac.
No sacrificing any of the audio information when ripping CDs that give you the perfect copies of your discs.
Safely convert to any other formats without any data or quality loss.
Decoding at a high speed.
Large file size that need more storage compared to lossy compressed files.
Limited support form hardware except Apple products.
Hardware: Apple iOS devices including iPod, iPhone, iPad and Mac.
Software: iTunes, QuickTime.
Comparison between ALAC and MP3
The difference between ALAC and MP3 is that whether they will lose some data in an encoding operation. MP3 is a lossy audio format which will cancel small parts of the high and low frequency of the audio data while ALAC as a lossless compression will retain all the initial stuff of the file.
Comparison between ALAC and FLAC
Both ALAC and FLAC files are the premium lossless formats and have nothing to do with audio quality.
They are different in that they use different schemes to achieve compression. Also, FLAC is open source yet ALAC is controlled by Apple. Hence, FLAC has great support everywhere else out of the Apple ecosystem.