Arch Linux is a rolling-release Linux distribution.
From the Arch Linux Wiki:
Arch Linux is an independently-developed i686/x86-64 community distribution, based on a rolling-release model and targeted at competent GNU/Linux users which offers large binary repositories and full-featured package management as well as a ports-like packaging system. Development focuses on a balance of minimalism, elegance, code correctness and modernity. Version 0.1 (Homer) was released March 11, 2002.
Arch is lightweight, flexible, simple and aims to be very UNIX-like. Its design philosophy and implementation make it easy to extend and mold into whatever kind of system you're building- from a minimalist console machine to the most grandiose and feature rich desktop environments available. Rather than tearing out unneeded and unwanted packages, Arch offers the power user the ability to build up from a minimal foundation without any defaults chosen for them. It is the user who decides what Arch Linux will be.
Arch Linux, unlike a typical linux distribution, does not include any default applications in its installation beyond basic utilities. Instead, it requires you choose what applications to install when you install Arch. With pacman Arch Linux has its own package management software.