Stark Tower is the main building of Stark Industries.
It is where Tony Stark do his other time as a normal businessman while not being Iron Man, and also where the Iron Man armory is.
The Arc Reactor is also placed there to provide clean and unlimited energy to the tower.
It appeared in theme song intro, other few episodes, etc. including in the Avengers EMH poser.
Tony Stark was in a meeting with Maria Hill when the Stark Tower was invaded by A.I.M.