what is the major difference between ROS distributions

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As with new Ubuntu distributions, new ROS distributions contain new features, some old behaviours or APIs get deprecated etc.

All in all, this is not easy to answer.

The three biggest differences are IMHO:

1. the latest distribution is the one that is actively developed (or at least supported --> LTS distros)
2. ROS distributions are only packaged for specific OS distros (e.g. Kinetic for Ubuntu 15.10 and 16.04, Lunar for Ubuntu 16.10 and 17.04, amongst others)
3. There has been one big change, the introduction of catkin between Fuerte and Groovy (if I am not mistaken).

More info on the wiki