Flake URL

A flake can be referred to using an URL-like syntax.

Examples

# A flake on a GitHub repo
github:srid/emanote

# A local flake at current directory
.

# Another way to refer to local flakes
path:/Users/srid/code/foo

# Full Git references is also possible:
git+https://github.com/juspay/services-flake?ref=dev

Reference

The URL-like syntax is documented here.

Linked from

Use a local directory as flake input

A Flake URL can not only be Git repositories. They can also refer to local paths. If you have two projects ~/code/foo and ~/code/bar, and bar depends on foo, you can use the following flake.nix in bar to have it refer to the local foo project:
Rapid Introduction to Nix

URL-like syntax used by the url attribute

The nix build command takes as argument a value of the form <flake-url>#<package-name>. By default, . (which is a flake URL) refers to the current flake. Thus, nix build .#cowsay will build the cowsay package from the current flake under the current system. nix build produces a ./result symlink that points to the Nix store path containing the package:
First steps with Nix

[!info] nix run nix run command will run the specified package from the flake. Here nixpkgs is the flake, followed by the letter #, which is followed by the package (Derivation) name cowsay that is outputted by that flake. See Flake URL for details on the syntax.

A registry is simply a shortcut mapping (like nixpkgs) to a full Flake URL (like github:NixOS/nixpkgs/nixpkgs-unstable).

You are not required to use a registry. Without a registry, getting a package off nixpkgs would instead involve its fully qualified URL:

nixpkgs is the main repository, but Nix allows installing software from anywhere. You can run or install programs from any flake by specifying its URL.