Short answers, pulled from the story.
A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping. Its boundary of no escape is called the event horizon, and general relativity predicts a central singularity inside where the curvature of spacetime is infinite.
English astronomer and clergyman John Michell first proposed the idea of a body so massive that light could not escape, in a letter published in 1784. French scientist Pierre-Simon Laplace suggested the same idea independently in his 1796 book Exposition du Systeme du Monde.
Cygnus X-1 was the first widely accepted black hole, identified by several researchers independently in 1971 after X-ray observations by Riccardo Giacconi's team. By 1974 it was widely considered a black hole, and in 2011 its mass was estimated at 14.1 solar masses.
Sagittarius A*, the radio source at the core of the Milky Way, contains a supermassive black hole of about 4.3 million solar masses. Astronomers established this by tracking the motions of over 100 stars orbiting the invisible object, including one star called S2 that completed a full orbit.
Hawking radiation is thermal emission that quantum field theory predicts event horizons produce, with the rate inversely proportional to the black hole's mass. Hawking showed this in 1974, but stellar black holes gain mass from the cosmic microwave background faster than they lose it through Hawking radiation.
Black holes are inferred through their interaction with matter and electromagnetic radiation, through gravitational waves from mergers, and through the motions of orbiting stars. The first direct detection of gravitational waves, GW150914, came in late 2015, and the Event Horizon Telescope published the first direct image of a black hole on the 10th of April 2019.
Every black hole has a singularity where spacetime curvature becomes infinite and all of its mass is concentrated in zero volume. An infalling observer is torn apart by growing tidal forces in a process called spaghettification, though alternative theories like the fuzzball model and loop quantum gravity propose black holes without true singularities.