ROS is an open-source, meta-operating system for your robot. It provides the services you would expect from an operating system, including hardware abstraction, low-level device control, implementation of commonly-used functionality, message-passing between processes, and package management.
With the ROS Fuerte release, it's much easier to get ROS running on an Arch system but still takes some effort. Read the notes below and then got to the installation page on the ROS website
With fuerte, there ROS is split into two levels. The core libraries are installed into the system folders (/opt/ros/fuerte) while the higher-level stacks and packages are installed in the user folders.
The Arch specific install page lists the packages (official and AUR) that you will need to install for a full setup.
There is a AUR package for the core libraries.
To build the higher level packages you will need to do some work...
- Install aur packages
ros-on-arch pcl-ros swig-wx qhull-ros
- Pull the sources with rosinstall
- Patch the sources to compile with GCC 4.7 
- Resolve dependencies with rosdep
- Build with rosmake