If you dont think about the big cities (Oslo, Stavanger, Bergen, Trondheim and Tromsø) the dialects changes very fast. Im from the eastern part of Norway, and if i drive 15 minutes i can arrive to a place where they use words I have never heard before. And we have three official languages/writings in Norway; Bokmål (South/east and the northern part of norway), Nynorsk (South/west) and the sami language (It origins from the northern part of Norway but since most of the sami people live in Oslo you can encounter some people that know how to speak and write sami there).
As a major dialect i would say that the easiest one to understand is the one they use in Oslo, thats most similar to Bokmål.
I can describe the eastern dialect with what i said above; it's similar to how you write bokmål. South and western dialect i dont even want to try explaining, except that for an untrained ear it can sound a little bit danish/dutch.
The northern part swears ALOT.