The history behind the multiple languages, and in particular the two "Norwegian" languages, is related to Norway's relationship with its neighbours.
For 436 years, until 1814, Norway was the weaker part in the Denmark-Norway union, making Denmark a considerable force in driving Norwegian policy, culture and language. Hence the Norwegian language became heavily "polluted" by Danish.
Since Denmark-Norway was on the loosing side of the Napoleon war, Denmark had to cede Norway to Sweden in 1814. Under the Swedish rule Norway was more independent than under Denmark, which eventually had an impact on Norwegian language; in the 1850's Ivar Aasen "created" a new Norwegian language (nynorsk) based on the spoken dialect in the rural areas of Norway (the Danish-inspired Norwegian was most common in the cities), and by late 19th century this language was acknowledged by the parliament.
Today the two biggest official languages of Norway are "bokmål" (the one influenced mostly by Danish) and "nynorsk" (the one "created" in the 1850s). "Nynorsk" has its foothold along the Norwegian west coast, mainly in the rural areas, and in the mountain/valley areas of central Norway.