Short answers, pulled from the story.
The International Astronomical Union created the Small Solar System Bodies category in August of 2006. This decision occurred when the organization gathered in Prague to vote on a new definition for planets.
Most small Solar System bodies reside within the main asteroid belt between Mars and Jupiter. This region contains classical asteroids that are not large enough to become dwarf planets.
Comets differ from asteroids by containing significant amounts of ice and volatile materials. When comets approach the inner solar system, their ices sublimate to create glowing tails which distinguishes them visually from rocky minor planets.
Interstellar objects like 1I/Oumuamua are excluded because they follow hyperbolic trajectories that allow them to escape the solar gravitational field. The IAU definition specifically limits Small Solar System Bodies to objects bound to our Sun.
Future definitions may establish a lower size bound based on whether objects have achieved hydrostatic equilibrium. If more large objects meet this criteria, the number of SSSBs could decrease significantly.