We can talk about history of this conflict if you want. But there are a lots of details and you have to spend a lot of time to learn them. I'm not sure you want this.
So. So called "Donbasniks" started coup against ukraine in april 2014. They claimed to be local fighters of the Donetsk People's Republic, but were actually Russian Armed Forces 'volunteers' under the command of Russian GRU colonel Igor Girkin ('Strelkov').
The same dude, Strelkov, participated in Crimea annexation. Later he said openly and bravery about all this. You can find interview with him in russian.
Probably all of Ukraine saw the video from Slavyansk in 2014 when one of the soldiers said the word [parebrik] (border). I, for example, heard this word for the first time, although I speak russian. It turned out that this word originated and used in the Leningrad region.
that is the video
https://www.youtube.com/watch?v=Q_eLCWI2sZQ
btw, isn't this is strange that "Donbasniks" rebels equipped like fucking professional marines?