[mightybyte]

NixOS is a Linux distro built around the Nix package system. Nix is built around the idea of immutability. It makes all packages immutable by giving them their own directory identified by a hash that is derived from ALL of that package's dependencies. This has a number of desirable properties:

* It makes it trivial to have multiple versions of the same package installed at the same time and allows you to switch between them at will.

* It is trivial to roll back your system after a failed upgrade. Difficult system recovers after you upgrade to a new unstable version are a thing of the past.

* Non-privileged users can install software completely securely.

* Projects packaged with nix have the best possible build reproducibility because nix accounts for ALL of your dependencies all the way down to the lowest level system libraries, compilers, etc.