Stars like our sun are violent creatures — constantly spewing out radiation, gamma rays and all kinds of nasty stuff (though luckily Earth's ozone layer and atmosphere protect us from the worst of it.) But when stars die, especially big ones, their wrath becomes even more merciless. Stars at the end of their life cycle that are sufficiently huge will collapse in on themselves, forming a black hole. These singularities are defined by their gravitational pull, which is so incredibly strong that nothing — not even light — can escape. In other words, what happens in a black hole stays in a black hole.
But black holes aren't too limited by size or number. On average, a standard black hole is about three to ten times the size of our sun. As massive as "regular" black holes can get, supermassive black holes — which likely form over billions of years as black holes merge — can reach a size millions or billions that of our nearest star. And the universe could be filled with billions of supermassive black holes. In fact, there's one at the center of our galaxy, the Milky Way, called Sagittarius A*, around which all things in the galaxy rotate.
