A dot-file is generally any file whose name begins with a full stop.
https://en.wikipedia.org/wiki/Dot-file
Why and what
https://martinfowler.com/bliki/PhoenixServer.html
The primary advantage of using phoenix servers is to avoid configuration drift: ad hoc changes to a systems configuration that go unrecorded.
NixOS has a completely declarative approach to configuration management: you write a specification of the desired configuration of your system in NixOS’s modular language, and NixOS takes care of making it happen.
https://nixos.org/
http://docs.ansible.com/ansible/latest/index.html
https://knpuniversity.com/screencast/ansible/
http://hakunin.com/six-ansible-practices
http://www.nickhammond.com/automating-development-environment-ansible/