Guix provides a command-line package management interface (see Package Management), tools to help with software development (see Development), command-line utilities for more advanced usage (see Utilities), as well as Scheme programming interfaces (see Programming Interface). Its build daemon is responsible for building packages on behalf of users (see Setting Up the Daemon) and for downloading pre-built binaries from authorized sources (see Substitutes).
Guix includes package definitions for many GNU and non-GNU packages, all of which respect the user’s computing freedom. It is extensible: users can write their own package definitions (see Defining Packages) and make them available as independent package modules (see Package Modules). It is also customizable: users can derive specialized package definitions from existing ones, including from the command line (see Package Transformation Options).
Under the hood, Guix implements the functional package management discipline pioneered by Nix (see Acknowledgments). In Guix, the package build and installation process is seen as a function, in the mathematical sense. That function takes inputs, such as build scripts, a compiler, and libraries, and returns an installed package. As a pure function, its result depends solely on its inputs—for instance, it cannot refer to software or scripts that were not explicitly passed as inputs. A build function always produces the same result when passed a given set of inputs. It cannot alter the environment of the running system in any way; for instance, it cannot create, modify, or delete files outside of its build and installation directories. This is achieved by running build processes in isolated environments (or containers), where only their explicit inputs are visible.
The result of package build functions is cached in the file system, in a special directory called the store (see The Store). Each package is installed in a directory of its own in the store—by default under /gnu/store. The directory name contains a hash of all the inputs used to build that package; thus, changing an input yields a different directory name.
This approach is the foundation for the salient features of Guix: support for transactional package upgrade and rollback, per-user installation, and garbage collection of packages (see Features).