.deb is the extension of the Debian software package format and the most often used name for such binary packages. Like the "Deb" part of the term Debian, it originates from the name of Debra, erstwhile girlfriend and now ex-wife of Debian's founder Ian Murdock.
Debian packages are also used in distributions based on Debian, such as Ubuntu and official derivatives of Ubuntu (Lubuntu, Xubuntu).
Technically, such a package is a .ar archive with a different file extension, possibly compressed, and containing a file called
CONTROL. This file includes important information, such as dependencies, recommendations, and conflicting packages. It will also include scripts (including a shebang to allow running under any executable). Four such scripts exist: Pre-install, post-install, pre-remove, and post-remove. The package(and the control file) must conform to the Debian policy manual. Many aspects of this policy can be automatically checked using a tool called Lintian.
Any question specifically relating to installing a .deb file should be tagged with this as well as software-installation, weather you are installing from the command line or through the software centre or similar GUI installation program, such as gdebi.
Any questions about the .deb format should be tagged with this, including (but not limited to) questions about why ubuntu uses it, how to make one and the way it works.
No questions about intalling via
apt-get or another system, unless it is installing off a .deb file that has been saved on the machine.