Hi,
Link given by @Kruse certainly is the starting to understand ros. Turtlesim is just to get you started on the various things that ROS has been built on which makes your life easier when you work on your projects. Various concepts that you may have to familiar are stacks, packages, topics, publishers, subscribers, parameters, services and responses. So you may test your understanding with turtlesim where its much easier to visualize whats happening. So please go through all the tutorials before you jump on to real projects.
Hope it helps,
Karthik