Short answers, pulled from the story.
The Oort cloud is a theorized shell of billions of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU (roughly 0.03 to 3.2 light-years). It encompasses a disc-shaped inner region aligned with the ecliptic, known as the Hills cloud, and a spherical outer region that encloses the entire Solar System. Both regions lie well beyond the heliosphere in interstellar space.
Dutch astronomer Jan Oort proposed the cloud in 1950 to explain why long-period comets continue to enter the inner Solar System despite the forces that destroy them over time. Estonian astronomer Ernst Opik had independently proposed a similar outer reservoir in 1932, which is why the structure is sometimes called the Opik-Oort cloud.
The Hills cloud is the inner, torus-shaped region of the Oort cloud, spanning roughly 2,000 to 20,000 AU from the Sun, proposed by Jack G. Hills in 1981. Models predict it is far denser than the outer cloud, holding tens to hundreds of times as many cometary nuclei. Unlike the outer cloud, the Hills cloud is more tightly bound to the Sun and has not acquired a spherical shape.
The gravitational pull of the Milky Way distorts the orbits of bodies at Oort cloud distances, where the Sun's gravity is weak enough to be influenced by galactic tides. Statistical models of long-period comet orbits suggest the galactic tide is the principal mechanism sending them toward the inner Solar System. Up to 90% of all comets originating from the Oort cloud may reach the inner Solar System as a result of this tidal influence.
In 1984, physicist Richard A. Muller proposed that the Sun has an undetected companion star, either a brown dwarf or a red dwarf, orbiting inside the Oort cloud. Named Nemesis, the object was hypothesized to pass through the cloud approximately every 26 million years, triggering comet showers. No evidence for Nemesis has been found, and recent scientific analysis no longer supports the idea that mass extinctions occur at regular, repeating intervals.
Voyager 1, the most distant human-made spacecraft, will not reach the Oort cloud for approximately 300 years. Once inside, it would take the spacecraft around 30,000 years to pass through the full extent of the cloud.