Pinning is the black art of Debian and its derivative distributions. Using pinning, you can set your preferences for which package repository to use, either for all installations or upgrades, or for a specific set of packages. Officially, however, Debian discourages pinning, because it can prevent package upgrades or even corrupt an entire system if used carelessly.

Debian, as you may know, uses three main repositories [1]. These are Stable, Testing, and Unstable, also known by their release names, which are currently Jessie, Stretch, and Sid – all characters from the Toy Story movies. A new package enters Debian in Unstable, and, when it meets certain requirements, moves to Testing. When a general release is made, the package moves to Stable. Between releases, StableUpdates and Backports are used to help keep Stable up to date with borrowings from Testing. Debian derivatives like Ubuntu organize repositories by other criteria but are still likely to have some repositories that are more stable or otherwise preferred.

However, both Debian's and Debian derivatives' main repositories are divided into sections based on licensing. The main section contains free-licensed packages, including core system components. By contrast, contrib contains free-licensed packages that depend on proprietary applications, while non-free contains proprietary packages. Debian installs with only main enabled, although contrib and non-free can be enabled by editing /etc/apt/sources.list and then running apt-get update.

[...]