Northern Vietnam gets fewer visitors than Ho Chi Minh City (Saigon) and the coastal cities in the south, but it's full of culture. Historically, and to this day, it has been significantly poorer than the southern part of the country. Cultural and political differences remain strong - the language is pronounced differently in some ways, and there's definitely a more authoritarian feel in the cities.
The largest city in northern Vietnam is the capital of Hanoi, and it's the only attractive one. But there are numerous other sites, such as Ha Long Bay, the mountain resort of Sapa, and the historical battlefield of Dien Bien Phu.