Nix package manager
Nix is a package manager and build system that parses reproducible build instructions specified in the Nix Expression Language, a pure functional language with lazy evaluation. Nix expressions are pure functions taking dependencies as arguments and producing derivation specifying a reproducible build environment for the package. Nix stores the results of the build in unique addresses specified by a hash of the complete dependence tree, creating an immutable package store that allows for atomic upgrades, rollbacks and concurrent installation of different versions of a package, essentially eliminating dependency hell.
Usage
Installation
The installation section of the Nix manual describes how to install Nix from binary or source on Linux and Mac systems for a user with root privileges; it is easiest to install Nix for a single user, but steps are also included for setting up a multiuser installation. For an extensive guide on various methods for installing Nix - including how to install Nix without root privileges - see the Nix Installation Guide.
Declarative Configuration
Support for declarative configuration in Nix is limited, and the solutions are somewhat fragmented. The only only configuration file officially associated with Nix is nix.conf (usually found in /etc/nix/), which defines a number of settings relating to how Nix, including build, garbage collection, sandboxing, and user permissions. Details on the available options are found in the section of the manual. Since most scenarios involve using Nixpkgs, configuration of ~/.config/nixpkgs/config.nix is often also helpful; here package overrides can be specified.
Work on Nix user profiles is well underway, see 
PR 9250.
Imperative Operations
To a large extent, package and environment management in Nix is imperative; user environments - including package installation and removal - is managed with the nix-env command, while nix-channels determine which version of [Nixpkgs] is used, and thus which version your packages will have.
User Environments
Nix offers an imperative package management command line tool - nix-env - which can be used to install packages at the user level. Packages installed using nix-env are only available to the given user, and do not change system state.
| Searching for packages | nix search packagename
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| Installing a package | nix-env -iA packagename
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| List installed packages | nix-env -q
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| Uninstall packages | nix-env -e packagename
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| Upgrade packages | nix-env -u
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Channels
Nix packages are distributed through a number of Nix channels: mechanisms for distributing Nix expressions as well as the associated binary caches for them. These channels are what determine which versions your packages have, and they can be broadly categorized into stable and unstable channels. Most users will want the stable channel, currently nixos-18.03. For more information on channels and how to choose them, see the Nix Channels article.
| Listing current channels | nix-channel --list
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| Adding a primary channel | nix-channel --add https://nixos.org/channels/channel-name nixos
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| Adding other channels | nix-channel --add https://some.channel/url my-alias
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| Remove a channel | nix-channel --remove channel-alias
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| Updating a channel | nix-channel --update channel-alias
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| Updating all channels | nix-channel --update
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Internals
Nix store
Packages built by Nix are placed in the read-only Nix store, normally found in /nix/store. Each package is given a unique address specified by a cryptographic hash followed by the package name and version, for example /nix/store/nawl092prjblbhvv16kxxbk6j9gkgcqm-git-2.14.1. These prefixes hash all the inputs to the build process, including the source files, the full dependency tree, compiler flags, etc. This allows Nix to simultaneously install different versions of the same package, and even different builds of the same version, for example variants built with different compilers. When adding, removing or updating a package, nothing is is removed from the store; instead, it symlinks to these packages are added, removed or changed in profiles.
Profiles
In order to construct a coherent user or system environment, Nix symbolically entries of the Nix store into profiles. These are the front-end by which Nix allows rollbacks: since the store is immutable and previous versions of profiles are kept, reverting to an earlier state is simply a matter of change the symlink to a previous profile. To be more precise, Nix symlinks binaries into entries of the Nix store representing the user environments. These user environments are then symlinked into labeled profiles stored in /nix/var/nix/profiles, which are in turn symlinked to the user's ~/.nix-profile.
Sandboxing
When sandbox builds are enabled, Nix will setup an isolated environment for each build process. It is used to remove further hidden dependencies set by the build environment to improve reproducibility. This includes access to the network during the build outside of fetch* functions and files outside the Nix store. Depending on the operating system access to other resources are blocked as well (ex. inter process communication is isolated on Linux); see nix.conf section in the Nix manual for details.
Sandboxes are not enabled by default in Nix as there are cases where it makes building packages harder (for example npm install will not work due missing network access).
In pull requests for Nixpkgs people are asked to test builds with sandboxing enabled (see Tested using sandboxing in the pull request template) because in official Hydra builds sandboxing is also used.
To configure Nix for sandboxing set build-use-sandbox = true; in /etc/nix/nix.conf; to configure NixOS for sandboxing set nix.useSandbox = true; in configuration.nix.