The DebianLinux Package format.

A .deb is an ar(1) archive that contains the files in the package plus some MetaData regarding how to install, what the package provides and depends on, etc. This data allows the APT tool to automatically handle dependencies.

dpkg(8) is the tool you use to manipulate these -- apt-get(8) calls dpkg.

See DebianPackaging for information on how to make one.