What is Hydra?

Hydra is, more or less, a build server. Unsurprisingly, it uses Nix and nixlang for specifying what to build. It's used to build the binary packages for nixpkgs, for example, as well as for running automated tests to ensure that packages actually work. If you're just using Nix or NixOS as an end user, you probably don't need to care about this.

Because Hydra supports deployment operations after a successful build-and-testing cycle, you could also technically consider it a Continuous Deployment system.