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.
Setup Notes - Groovy
Currently, groovy is the only (semi-)supported ROS release that has Arch packages. The core, comm, and robot variants have meta-packages that should install all the necessary packages:AUR, AUR, (TODO: robot metapackage). In addition, AUR (along with its dependencies) has a package. Beware, the dependency tree goes at least five levels deep at times. AUR handles it (somewhat), AUR falls flat. It is strongly recommended that you install AUR first to prevent yaourt's ridiculous dependency thrashing.
The packages are all available in the maintainer's github: arch-ros-stacks. Not all packages there are in a working state, so use with caution. Please, report any issues with the packages on their respective AUR pages.
Usage Notes - Groovy
Due to the fact that Ubuntu and Arch handle the python2/3 binary split differently, catkin_make[_isolated] will not work on its own. To use these commands, create the following aliases in your ~/.bashrc (or equivalent):
alias catkin_make='catkin_make -DPYTHON_EXECUTABLE=/usr/bin/python2 -DPYTHON_INCLUDE_DIR=/usr/include/python2.7 -DPYTHON_LIBRARY=/usr/lib/libpython2.7.so' alias catkin_make_isolated='catkin_make_isolated -DPYTHON_EXECUTABLE=/usr/bin/python2 -DPYTHON_INCLUDE_DIR=/usr/include/python2.7 -DPYTHON_LIBRARY=/usr/lib/libpython2.7.so'
Without these lines, cmake will default to python 3, which you don't want. It is also possible to simply add the defines into any call of catkin_make[_isolated] (which you have to do if you want to call those commands within a script).
For further information on how to use ROS, see the ROS Tutorials.