Black Hole

A black hole is an object with infinite density, any object entering its event horizon is "spaghettified" and pulled into the black hole. There are 2 main types of black holes, stellar black holes and supermassive/ultramassive/hypermassive black holes.

Stellar Black Holes
These black holes form when extremely massive stars go supernova turning into a neutron star, but it is so dense that its gravity pulls the neutron star down into itself, collapsing into a point with infinite density, a black hole. These black holes are usually big, but not too big compared to the other black holes...

Supermassive Black Holes
When the big bang just happended, stars begin to form with the mass from the big bang, but the universe was very very very small and mass was very dense, so the stars formed there would be very huge and burn out their fuel very quickly, forming a very massive black hole. These black holes are usually in the center of galaxies!

Quasars
A quasar is a supermassive black hole when there is a bunch of gas going into it, causing the mass to heat up so much that is gets so luminous/bright. These quasars are uncommon now.