
Perhaps because of historical reasons (the closed country period, the periods of internal strife and non-unified Japan), a lot of dialects in Japanese have survived and are still used today.
First of all, the dialect of Japanese that you learn abroad is probably 標準語
hyoujungo or 'standard Japanese.' It must be noted that this standard Japanese is by all definitions a dialect. It is the dialect spoken in Tokyo, the capital of Japan. This official status has granted it widespread use and understanding.
But don't be fooled. There are a lot of other dialects out there. Some are rather similar, largely differing only in the use of 語尾
gobi, the often emotive sentence ending particles. Others, though, inflect grammar differently (for example, maybe they negate verbs differently, or even different verbs altogether), and use different words to refer to the same things.
Some dialects are not so tough, and even a foreginer might be able to make heads or tails of them. Others will probably sound more like Korean to a beginner. Take 関西弁
kansai-ben, the language spoken around Kansai (think Kyoto, Oosaka, Nara). It's different, but not to the point where you'll break down crying.
On the other hand though, are the Ryuukyuu dialects, which use many, many different vocabulary items, inflections and gobi. Kagoshima-ben probably deserves a mention, in that it was apparently deliberately made difficult to master to deter spies in times of war.
Outside of Japan, and outside of their respective prefectures, it may be rare to encounter non-standard dialects of Japanese. One that has gotten a lot of media attention is Kansai-ben, which is actually broken down further into Kyouto-ben, Oosaka-ben, Wakayama-ben etc, each differing a little.
Interestingly, a few people have differing opinions on their dialects. Working at a Japanese restaurant, I get to encounter people from all over (though, notably, many from Kantou). Of course, when speaking formally, the Toukyou dialect is demanded, but the interesting part lies in casual conversation.
One guy talked flat-out in (what I think was) Kyouto-ben. It took me out of my comfort zone for a while, but it does grow on you. Another girl had dropped her native Awa-ben (Shikoku area?), for the Kansai variation.
Others still prefer to speak Hyoujungo over their own dialects. There may be psychological or cultural reasons for this, but I can't go into those based purely on observation.
One Kansai speaker, told me that he and apparently many other Kansai-jin feel that the Toukyou dialect has an air of arrogance, or the feeling that one is talking down to another. This, by contrast, would seem to imply that Kansai-ben is more friendly. This may be why Kansai Comedians are so damned funny?
Anyway. Check out
Wikipedia's page for more detail on Japanese dialects than I plan to go into, ever.
For those interested in Kansai-ben (I love it v___v), check out
this very detailed page, which takes a rather in-depth linguistic look at the major dialect.