What is ROS and not is best explained in this article .
To cast intodruction to ros.org:
It provides the services you expect from an operating system, including hardware abstraction, low-level device management, the implementation of commonly used functions, message passing between processes, and packet management. It also provides tools and libraries for receiving, creating, writing, and running code on multiple computers.
You will also find a good explanation on this wiki. The main difference is that the meta operating system is built on top of the operating system and allows various processes (nodes) to interact with each other at run time.