An archive containing a piece of software (be that in binary or source form) as well as MetaData about that software which enables the PackageManagementTool to fulfill its PackageManagement duties. This includes a human readable description of the packaged software, and in nearly all Package formats also a list of architectures it will run and a list of dependencies, ie other Packages that are required in order for the software in this one to run correctly. Many formats also have provisions for storing a maintainer contact, a checksum and/or a cryptographic signature of the Package, its license, and more.

The two most common formats are RPM (invented by RedHat and used by most LinuxDistributions) and Deb (designed for Debian; obviously also used by Debian-derived LinuxDistributions).

Slackware has the simplest a Package format: almost plain TarBalls of the installed software, the MetaData simply consisting of a file with a description of the package and a postinstall script. There is no information about dependencies. This is not necessarily a drawback; on simple setups, the fact that no (corruptible) DataBase of installed Packages needs maintaining can be desirable.

A link to a comparison of Package formats AsSeenOnSlashdot: http://www.kitenet.net/joey/pkg-comp/