North Sydney is an area of Sydney pretty much immediately north of, and linked with the Sydney CBD by, the Sydney Harbour Bridge.
The North Sydney CBD is, from memory, the 6th largest CBD in Australia, so it is a significant business district of its own right, not just an extension of the Sydney CBD. It has a major technology focus, and such companies as Optus (the second largest telco in Australia) and Cisco Systems, amoung many others, are based here.
North Sydney is also the heart of Sydney's affluent North Shore, with lots of impressive old houses around. It is surrounded by rich residential suburbs like McMahon's Point, Milson's Point, Lavender Bay, Kirribilli, Neutral Bay and Waverton.
(Yay, my first nodeshell rescue!)