(covers information from several alternate timelines)
Andor was an inhabited planet in the Kandari sector of the Alpha Quadrant. By the late 24th century, this Federation world was considered to be of comparable strategic importance as Berengaria, Vulcan, and even perhaps Earth. (Star Trek VI: The Undiscovered Country, production art; TNG: "Conspiracy", display graphic; DS9: "In the Cards").
Some time before 2363, as a junior officer, Lisa Cusak was assigned as an attaché to the Federation embassy on Andor. She met an Andorian civilian who worked at the Andorian Agricultural Ministry. The two ended up dating for 6 years. (DS9: "The Sound of Her Voice")
In 2373, Kai Winn rhetorically asked Captain Benjamin Sisko if the Federation would be willing to sacrifice Andor, Vulcan, Berengaria or even Earth to protect Bajor from the Dominion if necessary. (DS9: "In the Cards")
In 2374, one of the suggestions that Worf offered up for his honeymoon with Jadzia was a mountain climbing expedition on Andor. (DS9: "Change of Heart") After Betazed fell to the Dominion later that year, many believed Andor might next be threatened. (DS9: "In the Pale Moonlight")
In Star Trek: Enterprise's season 4 DVD, writers Judith and Garfield Reeves-Stevens have said they made the Andorian homeworld a satellite of a gas giant to help explain why both names Andor and Andoria have been used interchangeably. Andor was the gas giant and Andoria was the moon.
In Star Trek Maps, and Star Fleet Medical Reference Manual, Andor was referred to as the Andorian homeworld. According to Star Trek: Star Charts (page 52), Andor was the name of the capital city of Andoria.
The location of Andor in the Milky Way Galaxy was depicted on the Federation star chart, "The Explored Galaxy", seen in Star Trek VI: The Undiscovered Country and several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes. In the "The Explored Galaxy" star chart Andor was located between the Tholian Assembly and Romulus. Both locations were identified in DS9: "Call to Arms" as being located in the Alpha Quadrant.
According to Worlds of Star Trek: Deep Space Nine, Volume 1, the difference between Andor and Andoria was the same as the difference between Terra and Earth. Andoria and Earth being the names preferred by the natives.