Short answers, pulled from the story.
The Kuiper belt is a circumstellar disc in the outer Solar System extending from the orbit of Neptune at 30 astronomical units to roughly 50 AU from the Sun. It is similar to the asteroid belt but about 20 times as wide and 20 to 200 times as massive, made mostly of small icy bodies.
The Kuiper belt is named for the Dutch astronomer Gerard Kuiper, who speculated about a similar disc in a 1951 paper. The name was coined by Scott Tremaine because the words Kuiper and comet belt appeared in the opening sentence of Julio Fernandez's 1980 paper.
The first Kuiper belt object after Pluto and Charon was discovered on the 30th of August 1992 by David C. Jewitt and Jane Luu, who announced the candidate object 1992 QB1, later named 15760 Albion. It came after five years of searching with a blink comparator and CCD detectors.
Pluto is the largest and most massive known member of the Kuiper belt, sharing the same 2:3 orbital resonance with Neptune that defines a class of objects called plutinos. After Eris was found to be 27 percent more massive than Pluto, the International Astronomical Union reclassified Pluto as a dwarf planet in 2006.
Kuiper belt objects are composed of a mixture of rock and ices such as water, methane, and ammonia, kept solid by temperatures of about 50 K. Their densities range from less than 0.4 to 2.6 grams per cubic centimeter, and their surfaces range in color from neutral grey to deep red.
The Kuiper cliff is a sudden drop in the number of known objects beyond the 1:2 resonance with Neptune at about 47.8 AU. The falloff of objects 100 km or larger past 50 AU was confirmed as real by Bernstein, Trilling, and colleagues in 2003, and its cause remains unknown.
New Horizons launched on the 19th of January 2006, flew past Pluto on the 14th of July 2015, and flew by the object 486958 Arrokoth on the 1st of January 2019. Arrokoth proved to be a contact binary 32 km long by 16 km wide, with its red color confirmed by the Ralph instrument.