Arch Linux Arm is a distribution of the Arm Linux specifically designed to run on devices using an Arm processor such as the Raspberry Pi which uses the ARMv6 processor.
Arm Linux is not aimed at users who have not used a Linux operating system before as the initial installation just installs the minimal applications to get the system running and has no desktop, just the shell. If your not familiar with Linux, the shell is a powerful environment where you type commands to control the computer, no mouse required.
You will need to be competent with the Linux shell commands and use the package manager Pacman to get going. The user then installs the rest of the system as required like a desktop of your choice such as Gnome, KDE, LXDE and Xfce if you even want one. So if you need a simple barebones OS to run an electronics project or a fully featured desktop with the latest programs then the choice is yours.
The Arch concept is based on the idea of keeping it simple. Build your system from the base up not strip out unwanted pre-set features to gain performance. Though setting up an configuring a system is not a simple activity and can have a steep learning curve the Arch Linux system dosn't add any unnecessary additions, modifications or complications. Once your system is installed and working you won't need to worry about future upgrades and re-installations as there is a nightly update cycle of packages meaning that you can always be up to date with the latest stable versions of your programs.
Alot of the packages you find on other Linux distributions are available or there are good alternatives. To see what software is available you can search the packages at Arch Linux Arm Packages.
The Arch Linux Wiki is a good place to find out more, at wiki.archlinux.org
The Arm version is available at archlinuxarm.org/platforms/armv6/raspberry-pi
You will need a least a 2gb SD card to install Arch Linux Arm on the Raspberry Pi.
Some Examples of Arch Linux Desktops