Norway has many regions that due to geography have had little contact throughout the centuries until at least the early 20th century. As a result there are many local dialects and regions with quite distinct patterns.
I'm probably not unique when saying that even though English is my second language, it can be far less stressful to understand than a lot of Norwegian dialects. If you are somewhat familiar with Norwegian, compare these two dialects.
This is from area of Borre - a dialect very similar to standard Norwegian you will learn in my or most other classes:
The same story with a Bjugn dialect, which is not an extreme example.
The majority of Norwegians has a dialect that has some significant degree diverge from the "Standard".
Most commonly there are 4 groups of Norwegian dialects.
1. Eastern Norwegian
2. Western Norwegian
3. Northern Norwegian
4. Trøndelag dialect
In addition you can count the dialect of Southern Norway and Mid-Norway. However, as previously mention there can be some big differences within the regions.
For Example the Bergen dialect has many destinct caracteristics in comparison to nearby dialects. In Bergen you can avoid the female nouns, they use much more bokmål than many surrounding areas. In addition you can even use the definitive form for names. E.g. if your last name is "Dal", people might refer to you as "Dalen".
For courses in Norwegian:
Curso de Noruegues - Learning Norwegian