The Black Hole That Kills Galaxies - Quasars

The universe isn't just a vast empty ocean sprinkled with galaxies – most of the atoms are actually drifting in between, in the intergalactic medium.
If we look closely, we can see who is in charge here: Quasars, the single most powerful objects in existence. As small as a grain of sand compared to the amazon river, they reside in the centers of some galaxies, shining with the power of a trillion stars, blasting out huge jets of matter, completely reshaping the cosmos around them. They are so powerful that they can kill a galaxy.
What are they, and how do they mold the structure of the universe at their whim?