Coruscant only gets a mention in the first three films produced ( eps. IV, V, VI ) but plays a major part in the second trilogy ( I, II, III ). It is the heart of the Star Wars galaxy, the place where the Emporer Palpatine reigns from. Before the Emporer came to power, it was the home for the Jedi Council and the Grand Assembly, something like the United Nations building in New York.
The planet was created by Timothy Zahn for his Admiral Thrawn trilogy. Lucas liked the planet and it became central to the second three films. The scene set on Coruscant in the original series was added for the edited films in the late nineties.
The natural woodland and features had long gone before any of the stories that were set on the planet. It is not know what the original atmosphere and environment was, was the planet a desert world like Mars and then terrformed to be breathable or was it like Earth and just built over.