A Debian Package is a file that ends in .deb and contains software for your Debian system.
A .deb is also known as a binary package. This means that the program inside the package is ready to run on your system.
There are also source packages.
A Debian package is smart enough to know how to add itself to your system, remove itself, and even configure itself to your needs.
Every package comes with its own DebianPackageDocumentation.
What's in a name? Every package has a name, and there are certain rules for what can and can't be in it. See also PackageInstallTips.
The MIME type is
For a keyword search in the package descriptions, use apt-cache.
If you look specifically for alternatives to some Windows programs see WindowsEquivalent.
If you wonder where a package you've got installed on your system has gone, when it becomes "obsolete" and unavailable, see http://ftp-master.debian.org/removals.txt to find out about removed packages.
If you think your package is just plain broken please submit a Debian bug (see reportbug)!
How to extract the content of a package
Do not attempt to use just dpkg-deb or the other commands in this page to install software! You must use dpkg proper to ensure that all the files are correctly placed and the package's scripts run and its status and contents recorded.
dpkg-deb -x file.deb dir
www.debian.org/doc/debian-policy/ - Debian Policy Manual's Virtual packages
Sometimes, there are several packages which offer more-or-less the same functionality. In this case, it's useful to define a virtual package whose name describes that common functionality...
ToDo: This page needs cleanup