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FLAC (excerpt from wikipedia)

Free Lossless Audio Codec (FLAC) is an audio compression codec primarily authored by Josh Coalson and Ed Whitney. As its name implies, FLAC employs a lossless data compression algorithm: a digital audio recording compressed by FLAC can be decompressed into an identical copy of the original audio data. Audio sources encoded to FLAC are typically reduced to 50–60% of their original size.

Lossless Data Compression (excerpt from wikipedia)

Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange for better compression rates.

Lossless data compression is used in many applications. For example, it is used in the popular ZIP file format and in the Unix tool gzip. It is also often used as a component within lossy data compression technologies (e.g. lossless mid/side joint stereo preprocessing by the LAME MP3 encoder and other lossy audio encoders).

Lossless compression is used in cases where it is important that the original and the decompressed data be identical, or where deviations from the original data could be deleterious. Typical examples are executable programs, text documents and source code. Some image file formats, like PNG or GIF, use only lossless compression, while others like TIFF and MNG may use either lossless or lossy methods. Lossless audio formats are most often used for archiving or production purposes, with smaller lossy audio files being typically used on portable players and in other cases where storage space is limited and/or exact replication of the audio is unnecessary.

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There are a number of lossless compression codecs, you can find a large number of more technical explanations with a search engine and a little effort.......