Arch is a distribution of Linux, which centers itself around a philosophy of keeping
things simple. Like most other flavors of Linux, Arch distributes packages in a binary format,
optimized for i686 processors, which means Arch will only run on a Pentium 2 or higher processor. One important thing to note is that this is not the best choice for a new user, as you are essentially installing your system from scratch and almost no automatic configuration is done for you. This is, however, a decent way to learn about the workings of a Linux system.
The installer is curses based and is relatively intuitive. Each step is pretty clear, and if you need to make changes to configuration files (and you will), the files themselves contain clear comments, giving you some direction on what you need to modify. Arch does not contain helper applications for configuration like other distributions, so files will be edited by hand. Once the installer has finished and you have rebooted, you will end up with a command line install. However, if you follow the excellent documentation on the Arch website, you will be up and running in no time.
Pacman is the package management utility which handles installing, updating and uninstalling software packages and related dependencies. It also tracks what packages have been installed as well as what is available to be installed. Arch is different from most distributions because it does not use a set version of software packages for each release. Rather, a release is a snapshot of Arch at the time the release is made. For example, The Gimp, a graphics package is sitting at version 2.2 when Arch was installed. A new version, 2.2.1, is released, and Arch developers package it, test it and release it. During your next system upgrade, you'll find that The Gimp is updated to 2.2.1. This is referred to as a rolling release, where instead of waiting for Arch version 2007.09 for 2.2.1, you get it when it's been packaged up.
Arch can be downloaded from the official website, http://www.archlinux.org, which also hosts
various documentation, a wiki, and user forums. As of this writing, the current version of Arch is 2007.08-2.