Rosinstall is an SCM abstracton tool for holding ROS workspaces in sync.

From the rosinstall documentation:

Using rosinstall you can update several folders using a variety of
SCMs (SVN, Mercurial, git, Bazaar) with just one command.

That way you can more effectively manage source code workspaces.

The rosinstall package provides a Python API for interacting with a
source code workspace as well as a group of command line
tools. Rosinstall leverages the vcstools package for source control
and stores its state in .rosinstall files.

rosinstall was developed to help with the ROS software, but it has no
install dependencies to ROS. It offers support for ROS environments
and thus makes some assumptions about ROS being present at runtime,
but those can be easily removed and rosws provides all services even
when there is no ROS installed. The vision is for the bulk of
rosinstall to be a ROS agnostic tool one day.

WWW: http://docs.ros.org/independent/api/rosinstall/html