What is NixOps?

Nix (and NixOS) themselves only manage a single machine. If you want to manage multiple machines, especially if they are many servers, you can use a tool like NixOps - it's an 'orchestration tool' like Ansible, Chef, or Puppet, but with the guarantees of Nix. Like all of the other tools, you use nix-lang for specifying your systems.

If you're curious about what NixOS with NixOps does better than other orchestration tools, give this excellent article a read.

NixOps is only one of many deployment tools for Nix and NixOS, and it is mentioned here because it was the first one. If you only need to manage one or a few servers, there are many other options that are often simpler, such as morph. These will likely get their own wiki articles at a later time.