deb is the extension of the Debian software package format and the most often used name for such binary packages.
Debian packages are standard Unix ar archives that include two tar archives optionally compressed with gzip (zlib), Bzip2, lzma, or xz (lzma2): one archive holds the control information and another contains the program data.
The accepted program for handling these packages is dpkg, most commonly via other programs such as apt/aptitude, the Ubuntu Software Center, Synaptic or Gdebi.
Debian packages can be converted into other packages and vice versa using alien, and created from source code using CheckInstall or Debian Package Maker.
Some core Debian packages are available as udebs (“micro debs”), and are typically used only for bootstrapping a Debian installation. Although these files use the udeb filename extension, they adhere to the same structure specification as ordinary deb files. However, unlike their deb counterparts, udeb packages contain only essential functional files. In particular, documentation files are normally omitted. udeb packages are not installable on a standard Debian system.
Reference from Wikipedia.