Need some little help with an ROS Overview

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Hey there,

I am still pretty new to ROS. I already did several indtroduction tutorials and stuff and tried to get an overview. But what I still cant figure out is:

• Where is the link betwenn GAZEBOSim and ROS? I am maybe not far enough in tutorials but I am still wondering either i do tutorials in ROS or Gazebo not now together.
• I am using Ubuntu 14.04 so trusty thar, so the consequence is I have to use ROS Indigo which is not linkable to the newest GAZEBO distro (3.0 would be the newest and Indigo is just supporting 2.2) am I right there? Is there a way to get this and what would be the advantages.
• Is ROS Indigo already as much supported than Hydro or do you recommend me to go an Ubuntu version down to 13.x to work with hydro (there are also some books which are working with Hydro in which i am intrested in)

Please dont shake you head on this questions as I said i just started and wanted to get an overview .

Best Regards.