Data compression is a general term for a group of technologies that encode large files in order to shrink them down in size. The purpose is two-fold. Smaller files take up less room, leaving more storage real-estate. Also, smaller files are faster to transfer over a network, whether that network is the Internet, an intranet, or a local area network (LAN).

In the 1970s various techniques were available to archive files, or place them together in a single package to avoid sending multiple files between computers. The idea was soon augmented with data compression techniques, thus an “archive” is now used to describe a compressed file.

Data compression involves applying an algorithm to data that makes some of the repetitive bits unnecessary. You can think of it as a kind of shorthand map that gets stored with the compressed file. When decompressed, the map restores all of the missing bits, reconstituting the complete file. Data compression can be used with text, graphics, executable programs, and multimedia files, though some types of files compress better than others.

Today’s most commonly recognized data compression technique was originally used in the DOS operating system prior to Microsoft™ Windows™ becoming ubiquitous in the mid-90s. Author Phil Katz eventually termed these compressed files zipped files — the idea being that when the files were unzipped (decompressed), the full contents “popped” out. Files used with this data compression technique have the extension, .zip.

Files that are extremely large even when compressed can be split into pieces before being sent over a network. The pieces are collected and reassembled on the receiving end. The leading data compression technique for large files, also from the days of DOS, is called RAR, after author Eugene Roshal. Data compression programs that support Roshal ARchive files can create a set of RARs from a large multimedia file, for example, or decompress an existing RAR to reassemble a movie or program. These files have the extension .rar, or for multi-part files, part01.rar, part02.rar or .r01 r02.

Various music formats also use other flavors of data compression techniques to squeeze down music files while maintaining as much of the original quality as possible. The most obvious example is the .mp3 file. In this case however, the compressed file is not an archive and cannot be decompressed. The bits that are removed to achieve the smaller file size are gone for good. Other data compression techniques used for music files retain more quality but also result in less shrinkage of file size.

Data compression programs are widely available online. A zipped or RAR'd file requires a program that supports that type of data compression to unzip or unRAR it. Most data compression programs support multiple types of compressed files, and many of these programs are freeware.