A ZIP file contains one or more files that have been compressed to reduce file size, or stored as-is. The ZIP file format permits a number of compression algorithms, but as of 2009, the Deflate method continues to be dominant.
ZIP files generally use the file extensions ".zip" or ".ZIP" and the MIME media type application/zip, although the ZIP file format has also been used by many programs, usually under a different name. Examples of this usage are Java JAR files, Mozilla Firefox Add-ons (.xpi), id Software .pk3/.pk4 files, Activity Bundles (.xo) for the Sugar learning platform, Winamp and Windows Media Player skins, OpenDocument format and Office Open XML.
A ZIP file is identified by the presence of a central directory located at the end of the file. The directory stores a list of the names of the entries (files or directories) stored in the ZIP file, along with other metadata about the entry, and an offset into the ZIP file, pointing to the actual entry data.