Black holes are objects at the edge of human knowledge and comprehension.
“Black hole” is a term that humans use to represent an extremely compact region of the Universe: a region of spacetime where so much mass is enclosed in such a small volume, that the most compact object that we can imagine is formed.
Can you imagine squeezing the whole planet Earth between your fingers, until making it a tiny sphere of the size of a marble, or a human thumb? You would have then made the Earth compact enough to transform it into a black hole.
A black hole is a cosmic object so compact, that it defies our own concept of “compactness”, in fact, shaping reality in extremely fascinating ways.
What is compactness? It is a way of describing of how much mass is enclosed in a volume. The more mass you put into a given volume, the more compact object you get. The smaller you make the volume for a given mass, the more compact object you get.
Our spacetime (reality) is curved, bent by mass; the more compact an object is, the stronger bending of spacetime around it will exist. A black hole is a region of spacetime so compact, that the gravitational pull it exerts close to its boundaries is infinite, and not even light can escape from it. Around a black hole, spacetime is infinitely curved.
The “core” of a black hole is in fact a spacetime singularity. A singularity is an infinite point, a divergence: a place where the human mind cannot clearly go. Our spacetime is curved, and more and more curved by the more mass there is. The spacetime around a black hole is so curved that not even light is able to escape: all is bent.
Black holes have no solid surfaces: their boundary is called “Event Horizon“. The Event Horizon is the locus of spacetime from where the gravitational pull of the black hole does not allow to escape any object, not even light. Therefore everything that enters a black hole, stays into the black hole.
That is why we call these objects “black holes”: because we cannot see anything coming from them, not even light; and because anything getting close enough, cannot escape from them. But black holes are not actually “black”: they have no color, or they have all the colors; we just cannot see them, by definition (colors are transmitted by light, they are light). Black holes are not even “holes”: they are rather infinite spikes of density in the spacetime thread. This is why they can merge, and form new, bigger black holes, contributing to shaping the Cosmic Structures as we see them.
All in all, black holes are simple objects. They all resemble each other, and can be described by only three properties: their electric charge, their rate of rotation (spin), and their mass; nothing more! Furthermore, quite luckily for humans studying black holes, in the Universe we can neglect the charge of black holes, and focus instead on the rate on which they rotate, and especially (as it is easier to measure): on their mass.
The size of a black hole mainly depends on its mass: the Event Horizon of a black hole with the mass of planet Earth is a couple of centimeters large; but for a black hole with the mass of our Sun the Event Horizon is as large as a terrestrial city; and the largest black holes we know are as large as the whole Solar System: they contain a mass equivalent to billions times the mass of our star, the Sun.