This article deals with Sol as it relates to NationStates. For more general information, see the Wikipedia article on this subject.
Sol is the name of the Sun, as originally used in Latin. The planetary system orbiting it is often referred to as the "Sol System" but when one is speaking in terms of multiple systems, just "Sol" can also refer to the entire Solar System. (For more on naming practices, see also: Terra, Sol, and Luna)
The Sol system
The planets of the solar system are those nine bodies traditionally labelled as such: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto although Pluto is not universally recognized as a true planet. The largest moons in the solar system (those bigger than about 3000 km across) are Earth's Luna, Jupiter's Galilean moons Io, Europa, Ganymede, and Callisto, Saturn's moon Titan, and Neptune's captured moon Triton. Other moons that are important on account of being inhabited are Jupiter's Thebe and Themisto and Pluto's Charon.
The Sol star (the Sun)
Sol is believed by most to be a lifeless object, a fusion reactor producing the energy needed in order for life to bloom in the rest of the system. However, The Caloris Basin claims to have made contact with an ancient intelligence of plasmatic beings living in and on the sun. Dubbed Solar Giants (Or, in a more informal tone, Suncritters), this nation is effectively inhabiting the sun.
Another object could be the commune of Absolute Light in very low Solar Orbit, which can effectively be counted as being a part of the sun's political structure.