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File compression is any of various techinques to reduce the size of computer files. Size refers to the number of bytes, and not any physical size characteristics which may be known to the file such as page size.

Compression techniques often rely on sophisticated mathematics and the detection of recurring patterns and sequences which are replaced by simpler (and shorter) references. Various techniques are optimized for differing files and much of this is derived from a set of standards called LZ77 and LZ78, published by Abraham Lempel and Jacob Ziv.

Different techniques are often referred to as "codecs", which stands for "compressor/decompressor" or "coder/decoder" depending on who you ask.

General File Compression

Some formats, such as the UNIX specific Gzip and Bzip2 and the Windows specific ZIP are designed to work moderately well with many different types of files. Zip is also capable of archiving multiple files and directory trees into a single compressed file. (UNIX prefers the seperate program tar(1) for this purpose)

Audio Compression formats

In addition to standard compression, most types of audio compression work by removing parts of the sound wave that cannot be readily perceived by the human ear. This makes most types of audio compression fundamentally different from general compression formats which cannot loose any data.

The most popular audio codec is MP3, which stands for Motion-Picture Experts Group 2 layer 3. MPEG is a industry group which is connected to the MP3 standard. MP3 is almost synonymous with digital music, although a rival Apple-backed codec called AAC has made a few inroads and a rival (and markedly inferior) Microsoft codec called Windows Media Audio has enough market share that most players support it.

All of these technologies are patented and efforts to create a nonpatented audio codec have been largely unsuccessful, although the VORBIS codec is complete, technically superior and published, it has utterly failed to catch on.

Video Compression

Video compression is a rapidly evolving field and many new technologies have recently burst onto the scene, in particular the H.261-H.265 families and several related but incompatible MPEG codecs, the most popular of which is MPEG-4 part 2.

Due to the explosion of new high-definition video content, and the creation of high resolution telecines for television and video the quality of video compression codecs has vastly improved.

The codec used on DVDs is MPEG2-2 and the codec used on Blu-Ray discs is either MPEG2-2, an H.264 variant or VC-1.

(The term MPEG-4 encompasses a suite of standards and is not one single file-format, please see Wikipedia:MPEG-4)

Image Compression

Images typically are found in PNG or JPEG-JFIF format. The PNG codec is lossless, the decompressed data is identical to the compressed data, and the JPEG format is lossy, some data is lost in the compression process, this means that from a visual-results and file-size standpoint, PNG is suited to line drawings and other circumstances where distortion is unacceptable and JPEG to photographic images