The ROS distributions are very similar to Linux distributions, that is, a versioned set of ROS packages. Each distribution maintains a stable set of core packages, up to the End Of Life (EOL) of the distribution. 

The ROS distributions are fully compatible with Ubuntu, and most of the ROS distributions are planned according to their respective Ubuntu versions. 

The following are some of the latest ROS distributions (at the time of writing) that are recommended for use from the ROS website (http://wiki.ros.org/Distributions): 

The latest ROS distribution is Melodic Morenia. We will get support for this distribution up until May 2023. One of the problems with this latest ROS distribution is that most of the packages will not be available on it because it will take time to migrate them from the previous distribution. If you are looking for a stable distribution, you can go for ROS Kinetic Kame because the distribution started in 2016, and most of the packages are available on this distribution. The ROS Lunar Loggerhead distribution will stop being supported in May 2019, so I do not recommend that you use it.