7-Zip is an open source file archiver designed originally for Microsoft Windows. 7-Zip operates with the 7z archive format, and can read and write to several other archive formats. The program can be used from a command line interface, graphical user interface, or Windows shell integration. 7-Zip began in 1999 and is actively developed by Igor Pavlov. It is related to a cross-platform port, p7zip.
7-Zip is distributed under the GNU Lesser General Public License. It was the winner of the SourceForge.net 2007 community choice awards for "Technical Design" and for "Best Project".

The 7z archive format
Main article: 7z
By default, 7-Zip creates 7z format archives with a .7z file extension. Each archive can contain multiple directories and files. As a container format, security or size reduction are achieved using a stacked combination of filters. These can consist of pre-processors, compression algorithms, and encryption filters.
The core .7z compression uses a variety of algorithms, the most common of which are bzip2, LZMA2, and LZMA. Developed by Igor Pavlov, LZMA is a relatively new system, making its debut as part of the 7z format. LZMA consists of a large LZ-based sliding dictionary up to 4 GB in size, backed by a range coder.
The native 7z file format is open and modular. All filenames are stored as Unicode.
The official 7z file format specification is distributed with the program's source code. The specification can be found in plain text format in the doc\ subdirectory of the source code distribution.

Other supported formats
7-Zip supports a number of other compression and non-compression archive formats including:
Packing/unpacking of ZIP, gzip, bzip2, tar and, in betas for version 9, xz.
Unpacking only: Microsoft cabinet (CAB) files, RAR, MSLZ, SWF, FLV, ARJ, Z, LHA, cpio, smzip, JAR, ISO CD/DVD images (7Zip version 4.42 and up), DMG, HFS, rpm and Debian deb archives.
According to the 7-Zip website, since version 4.65 (from 2009-02-03), 7-Zip can unpack the following formats in addition to the formats it fully supports: ARJ, CAB, CHM, cpio, DEB, DMG, HFS, ISO, LZH, LZMA, MSI, NSIS, RAR, RPM, UDF, WIM, XAR and Z.
7-Zip is able to open some MSI files, allowing access to the meta-files within along with the main contents. Some Microsoft CAB (LZX compression) and NSIS (LZMA) installer formats can be opened, meaning that 7-Zip can be used to check if a binary file is an archive. Similarly, some executable programs (.EXEs) may be opened as archives and have their contents extracted by 7-Zip.
Since the 7-Zip 9.14 beta, 7-Zip can open and view the content of virtual hard disks.
When compressing ZIP or gzip files, 7-Zip uses a home-brewed DEFLATE encoder which is often able to achieve higher compression levels than the more common DEFLATE implementation of zlib, at the expense of compression speed. The 7-Zip deflate encoder implementation is available separately as part of the AdvanceCOMP suite of tools.