rosws is part of the rosinstall package. See py-rosws-stacks.
rosws was designed to setup and manage rosbuild workspaces. If you are building catkin workspaces then you need to use wstool instead.
For the new rosws tool see the documentation at: http://www.ros.org/doc/independent/api/rosinstall/html
UPDATE: The tool formerly called rosws described here is named py-rosws-stacks since rosinstall 0.6.0. See /py-rosws-stacks