Films always make it seem like speaking a language means you can go anywhere in that region and speak fluently, but often there are significant barriers between dialects, even dialects that are mutually intelligible. Believe me, as a legal translation pro I know that “mutually intelligible” is a phrase that often means a lot more work for me!
East and West
The dialects in Finnish are broadly divided into East and West. Unlike some other languages, these dialects are all mutually intelligible to a very high degree – no one who speaks Finnish has any trouble understanding any of these dialects. The differences are mainly in certain vowel sounds and the rhythms of the language.
As a result when travelling around Finland people often have the experience of being slightly disoriented and confused when they first encounter a dialect, and then suddenly “snapping into” the new rhythm and finding it perfectly comprehensible. The best metaphor I can offer for those who only speak English would be if you travelled to the Deep South of the United States and heard someone speaking in a thick accent – for the first minute you might think it was a foreign language, until your ear “caught” the accent, and then you would be fine.
The South-West dialects of Finnish resemble Estonian, of all things, with their main identifying feature being the abbreviation of the vowel sounds in the last words of sentences. They are spoken in Finland Proper and Satakunta. Finland Proper, by the way, is the name of a small province in Finland; many English-speakers think it refers to the whole country. Back in ancient times the Finns were just one of many tribes living in the area, and this province roughly corresponds to what used to be the Finns’ land, thus the ancient name. The Eastern dialects have retained (or re-acquired) the palatalization that Finnish has otherwise lost, an ancient link to the Uralic languages of its heritage.
The story of Meänkieli is interesting. As a dialect, it’s very close to standard Finnish, and yet it often is given the status of a separate language – for example, in Sweden it is an official minor language, just like Finnish, even though they are almost identical! This stems from the political separation that occurred in the 19th century, when this area of Finland was annexed by Russia. For some time the people in this area were cut off from Finland, and their language development progressed quite separately. However, the Meänkieli dialect is like other Finnish dialects: Very subtle in its differences, and completely mutually intelligible with standard Finnish and other dialects.
Image by Zakuragi [CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons