Short answers, pulled from the story.
The solid core of a comet is a dark, porous lump of rock and dust that reflects less than four percent of the light that strikes it. This nucleus contains water ice, frozen carbon dioxide, carbon monoxide, methane, and ammonia mixed with rocky particles. Modern observations describe this structure as deep fried ice cream, where a dense crystalline crust covers a colder, less dense interior.
The ion tail, or type I tail, is made of gases ionized by solar ultraviolet radiation and is pushed directly away from the Sun by the solar wind. The dust tail, or type II tail, consists of larger dust particles that form a curved structure following the comet's orbital path. These two tails point in different directions and are created by the interaction of solar radiation and solar wind plasma with the comet nucleus.
For most of human history, comets were seen as terrifying omens of death and disaster rather than celestial bodies. The fear of comets as acts of God and signs of impending doom was highest in Europe from 1200 to 1650 CE. In 1910, erroneous newspaper reports inspired panic that toxic gas cyanogen in the tail of Halley's Comet might poison millions.
Short-period comets with orbital periods of less than 200 years originate in the Kuiper belt or its associated scattered disc beyond the orbit of Neptune. Long-period comets with periods ranging from 200 years to millions of years are thought to originate in the Oort cloud. The Oort cloud occupies a vast space starting from between 2,000 and 50,000 astronomical units from the Sun.
The term dirty snowball was coined by Fred Whipple in 1950 to describe the composition of comets. This model proposed that comets were icy objects containing some dust and rock rather than rocky objects containing some ice. The dirty snowball model became accepted and was supported by observations of spacecraft that flew through the coma of Halley's Comet in 1986.
The Rosetta mission revealed that the nucleus of 67P/Churyumov, Gerasimenko has no magnetic field. On the 12th of November 2014, the Philae lander successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which were detected for the first time on a comet.