Nix stores packages in the Nix store, usually the directory /nix/store, where each package has its own unique subdirectory such as /nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1/, (it’s a cryptographic hash of the package’s build dependency graph).

Nix package manager features:

• Multiple versions;
• Multiple users: Each user can have a different profile, a set of packages in the Nix store that appear in the user’s PATH. If a user installs a package that another user has already installed previously, the package won’t be built or downloaded a second time.

Commands

nix-collect-garbage: This deletes all packages that aren’t in use by any user profile or by a currently running program.

Reference:

The Official Documentation