An archive containing a piece of software (be that in binary or source form) as well as MetaData about that software. This is at least a human readable description of the packaged software. In nearly all Package formats it also extends to a list of architectures the particular Package will run on, and a list of other Packages are required in order for the software in this one to run correctly. These are called dependencies. Many Package formats also have provisions for storing a maintainer contact, a checksum and/or a cryptographic signature of the Package, its license, and more. Together, these pieces enable the PackageManagementTool to fulfill its PackageManagement duties.
Slackware has the simplest 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 (such as many dedicated servers use), the fact that no (corruptible) DataBase of installed Packages needs maintaining can be desirable.