rosbuild vs catkin

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Both are buildsystems, meaning they generate executable files from source files. Both are meant to build many individual projects in the right order with a single command (The main difference to other build systems and the reason ROS has a buildsystem of its own). With both, cmake does most of the work (with rosbuild that's the typical usage), whereas the catkin or rosbuild code itself mostly coordinates multiple builds.

You could use google to find pages describing both systems. Their main differences are described here: http://www.ros.org/wiki/catkin_or_rosbuild