Developed in large part by now defunct Willow Garage, ROS was designed for collaborative, open source robotics development. ROS is a collection of tools and libraries that simplify the task of creating and programming robotic platforms and applications. ROS is not a real-time OS, but it can be integrated with RTOSes, as well as Linux. As of earlier this year, Android support was added.
The default install for ROS is Ubuntu Linux. Core ROS components include message passing, message recording and playback, remote procedure calls, and a distributed parameter system. In addition to these core middleware components, ROS offers more robotics-specific features like a Unified Robot Description Format (URDF), a remote geometry library, preemptable remote procedure calls, and diagnostics. It also offers ready-built packages for common robotics problems like mobile navigation, pose estimation, and building a map and having the robot self-localize on it.