Nanjing Lu (Nanjing Road) is a shopping street with a history. When the British began trading in Shanghai after the Opium Wars of the 19th century, this was one of their commercial centers. In those days it was called Nanking Road, the 'k' being a more popular form of Anglicization.
Today it's not only the hectic heart of Shangai's shopping and tourism scene, but the longest shopping district in the world (6 km/3.5 mi of go-go-go). Even back in its earliest days when it served the International Settlement, it quickly became dominated by big department stores. The chains have well and truly moved in here, but really it's for the experience as much as anything that you should come.
The street is divided into two lengths, east and west. The eastern section is pedestrianized. Avoid the Western generic stuff and concentrate on what are still the Shanghai specialties - silk, jade, clocks. If it all gets a bit much, there's a sight-seeing trackless train that can take you the length of the street at night.
The Eastern section of Nanjing Road runs from The Bund to People's Square. The Western section runs from People's Square to the Jing'an Temple. You can take Line 2 of the subway and get off at either East Nanjing Road station or West Nanjing Road station.