Debian, also known as “the universal operating system”, is one of the oldest Linux distributions. At any point in time there are always three main Debian releases: stable, testing and unstable. The “stable” release represents the official Debian release: it is rock solid, ready for production, and contains packages which doesn’t change much. The “testing” release contains packages which are on their road to be accepted into stable, and finally, the “unstable” release is the one with the most updated versions of software, used for the distribution development.
RPM is the acronym for Red Hat Package Manager: we use it to reference both the software package format and the low-level package manager used by the Red Hat family of distributions. Since version 4.12 of the latter it is possible to declare packages “weak dependencies”, which are installed by default, but not strictly required.
Developed by Canonical, the company behind Ubuntu, and originally meant to be used on the latter, the Snappy package manager is a free and open source software used to install and manage snap packages. The purpose of Snap packages, just like flatpaks, is to distribute sandboxed and self-contained applications (applications are packaged together with their dependencies).