Nintendo Network ID is a unique identifier made up of 6 to 16 characters that allows you to access Nintendo Network services, such as Miiverse and Nintendo eShop, on Wii U.
As of 10th December 2013, it is also possible to register a Nintendo Network ID on Nintendo 3DS.
If you have connected your Wii U to the Internet, we strongly recommend you register the same Nintendo Network ID on Nintendo 3DS. Using the same Nintendo Network ID on both systems will combine their Nintendo eShop balances.