Terminology

Objects orbiting the Sun are divided into three classes: planets, dwarf planets, and small Solar System bodies.
A planet is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape and b) has cleared its immediate neighborhood of all smaller objects. There are eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
On August 24, 2006 the International Astronomical Union defined the term "planet" for the first time, excluding Pluto and reclassifying it under the new category of dwarf planet along with Eris and Ceres.[2]
A dwarf planet is not required to clear its neighborhood of other celestial bodies. Other objects that may become classified as dwarf planets are Sedna, Orcus, and Quaoar.
From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. But in the late 20th and early 21st centuries, many objects similar to Pluto were discovered in the outer Solar System, most notably Eris, which is slightly larger than Pluto.
The remainder of the objects in orbit around the Sun are small Solar System bodies (SSSBs).[3]
Natural satellites, or moons, are those objects in orbit around planets, dwarf planets and SSSBs, rather than the Sun itself.
A planet's distance from the Sun varies in the course of its year. Its closest approach to the Sun is called its perihelion, while its farthest distance from the Sun is called its aphelion.
Astronomers usually measure distances within the Solar System in astronomical units (AU). One AU is the approximate distance between the Earth and the Sun, or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 38 AU from the Sun while Jupiter lies at roughly 5.2 AU. One light year, the best known unit of interstellar distance, is roughly 63,240 AU.
Informally, the Solar System is sometimes divided into separate zones. The inner Solar System includes the four terrestrial planets and the main asteroid belt. Some define the outer Solar System as comprising everything beyond the asteroids.[4] Others define it as the region beyond Neptune, with the four gas giants considered a separate "middle zone".