Here we will look at some salient and major variations in language use in Japan. In the limited space given to this essay, then, we can characterize, in certain broad strokes, only some of the factors that figure into variation in language use, that is, the variation due to region, gender, modality (spoken and written language), and age. We will have more to say about an important aspect of language variation—language of honorification—in a separate essay.
A dialect area is defined as a geographical area into which language variations systematically converge. Even within a dialect area, there are a number of discernibly different dialects. Depending on how finely one make a distinction between one dialect and another, one can say we have tens or as many as hundreds of dialects. Many factors have contributed to the rise of a plethora of dialects, a phenomenon comparable to the situation in the British isles. In Japan, dialectal variation is due to high mountains and wide rivers that constituted a natural barrier to communication, a relative immobility of people's lifestyle until recently, a long history of habitation, a lack of means of mass communication until recently, among others.
If we remove Okinawa (where a separate language is spoken) from our present discussion, there are three major dialect areas in Japan—the southern island of Kyūshū, eastern Japan (roughly east of Nagoya), and western Japan (roughly between Kyūshū and Nagoya)—with the dividing line (in linguistics, called an 'isogloss') between east and west in the middle of the Japanese main island of Honshū, demarcated by a range of mountains.
To illustrate the dialectal difference, let us contrast Tokyo-area Japanese and the variety spoken in the Osaka area. Keep mind that there are many dialects within the Eastern variety (Tokyo, Nagano, Chiba, etc.) and Kansai variety (Kobe, Osaka, Sakai, etc.), a fact sanctioned by the very dynamic definition of dialect indicated above, but we can nonetheless identify certain consistent differences between these two dialect groups:
East - West - Meaning
imperative form: yome - yomii - 'read'
past tense: katta - kōta - 'bought'
adverbial form: ōkiku naru - ōkyūnaru - 'become larger'
negative: sinai - sen - 'don't do'
copula: da ja, - ya - 'be'