Complicated and long story.

Putin's private army.

Wagner is a private military group set up as a private army that does what Russian army can't: die without being official numbers. And fight hard.

Many contracts from state budget.

They kicked American ass in Syria and seized oil fields there.

They own Central African Republic.

They kicked French ass in Mali.

They own mines in Africa and oil in Syria.

They took Crimeea for Putin.

They fought hard in Ucraine.

Their leader was in an open coflict with army generals and now that conflict reached the climax.

Putin sided with the generals.