Black Hole is massive collection of mass in a tiny space. Hence it has massive gravitational field. Even the fastest particles (Photons) can't escape this gravitational field. The only way to identify Black Hole is effects of it's gravitational field to surrounding objects. The dark area around Black Hole is a event horizon. If you toss some objects to a Black Hole, it's getting accelerated due to it's gravitational field. At a certain point, it's speed equal to speed of light. That point is the Event Horizon. When an objects reach the sped of light, it's flow of time will be frozen, After that all the laws of physics will be irrelevant. So we have no idea what is beyond this event horizon. And singularity is way event horizon. Even to date we don not have accurate way to identify it's composition.
So what is the origin of this massive collection of mass creating a black hole?
All the stars in this universe are made with Hydrogen. Nuclear fusion is fusing them to make helium and radiating massige amount of energy. That radiation is pushing thr Hydrogen away and gravity is pulling in. A star has spherical shape due this gydrostatic equilibrium. If this a massive star, Heliim could be fused to bigger elements and process goes further till they create Iron (Fe) and they are getting depostied in the core.
Down the time more Hydrogen getting fused and radiation is getting reduced. But gravity remains the remains the same. At a certain point gravity exceed the pressure of radiation, and the star collapsen in to it self in few seconds. Then star goes to brutal Supernova explosion. This is spreading out elements that cooked in the star. That how everything arround us, ecen humans are created. It will become a Neutron Star. If it's massive, it will create a singularity and then Black Hole.
Due to massive gravitational field, Black Hole can suck near by objects, Even light. So you must be thinking it's infinite. But even black holes have a finite life time. Through out the universe there are virtual particles. They are in pairs with + and - mass. In very short time they annihilate each other to pure energy. If there is a virtual pariticle pair near event horizon, Particle with + mass might get escaped. Particle with - mass get sucked in and that will reduce the mass od singularity. Escaped particle will become a radiation. This is a theory by famous Stephan Hawking. So we call it Hawking Radiation. But it's taking trillions and trilliond of years to evaporate a Black Hole. There are several sizes of black holes. Most of them are in size in of small asteroid. Some of them are couple of times bigger than solar system. We call them Supermassive Black Holes.
They live in the center of galaxies holding it's stars together. What is beyond event horizon? What is compostions of singularity? Those are still mysteries!