Coruscant was The Capital of the Old Republic, the Galactic Empire, the New Republic, and the Yuuzhan Vong occupation at various times.
It was a huge city covering the entire planet. New buildings were built on the old. As a result, there was virtually no exposed land.
The concept of a city covering an entire planet is not entirely new. The planet Trantor in Isaac Asimov's Foundation novels is probably the first fictional planet to be totally urbanized, but it was not the last.
The name itself is literary English meaning sparkling or glittering and probably refers to the night side of the planetwide city. Traditionally pronounced with a hard 'c' - ko'-rus-kant' - the word is pronounced Ko'-ru-sant in the Star Wars Universe.
Originally the planet, which was than called Alderaan, was to appear in the first Star Wars film, but the budget wouldn't allow, so a lot of action on Alderaan was moved to the Death Star and Alderaan became the name Princess Leia home planet which is destroyed. The concept of seeing the Empire's home world came up again in the Return of the Jedi, and the concept the entire planet being a city came up for the first time. However, realizing such a city on screen was imposible at the time so the idea was abandoned.
The planet first appeared in the Expanded Universe and was called Coruscant for the first time. Coruscant wasn't originally seen on screen until The Phantom Menace. It was seen on the screen in the 1997 Special Edition release of Return of the Jedi.
In the New Jedi Order series, Coruscant is the capital world of the New Republic until Star by Star, in which the extragalactic Yuuzhan Vong overwhelm the Republic defenses in three attack waves and take over the planet. The Vong designated it Yuuzhan'tar after their homeworld and terraformed it to overwhelm the cities covering its surface and restore a natural ecology.