[rcxdude]

It's a package manager written in such a way as to properly solve the problems with existing package managers which docker patches over. It uses deterministic builds and careful isolation of dependencies to ensure that the environment it creates is the same each time, and that you can have packages which depend on conflicting versions of another package installed at the same time.

The entire OS is accurately described by a config file, and this can be reproduced exactly using just that file.

In contrast to traditional package managers: handles conflicting dependencies, state is tracked through editing the config file, not a serial of install/uninstall commands which mutate the state of the system. Config files of installed packages are also controlled through nix config.

In constrast to docker: properly reproducable (Docker will re-run the same commands in the Dockerfile, but there's no guarantee you'll get the same result. For example, basically any package installation from a traditional package manager you run in the Dockerfile will not reproduce when run later because newer versions of packages will be installed), also more efficient in terms of space usage. However, AFAIK it does not namespace networking and so on (nixOS has its own containers system which does do this however).