Once you've struggled to master the very basics of child-level conversation in Japanese, you'll be distraught to learn that like all languages, it has its variations.
Different dialects exist across Japan. Japan is a very old country and some of these dialects have been separated from others for centuries. Like the rest of the world, the advent of TV has standardized the language for the most part, but these dialects still exist and if you travel around Japan, you'll come face to face with them.
Major Japanese dialects
Kanto-ben (関東弁 – Kanto dialect) is the Tokyo dialect, which is generally considered to be standard Japanese, although not technically so (ben means dialect). It's called standard largely because Tokyo is where much of Japan's mass culture is produced, being the hub of television, radio and so on.
Kanto-ben tends to be flat without much intonation. Among speech characteristics of Kanto dialect, there's the sa interjection interspersed throughout sentences, the -be and -ppe ending which also appears in Tohoku dialect, the change from –te iru to –tenno, and jyan at the end of sentences, which is a shortening of janai. If you're studying Japanese, what you're probably learning is Kanto-ben.
The second major dialect of Japan is Kansai-ben (関西弁), which is spoken in Osaka and Kyoto and throughout the south-western area of Honshu. Technically, Kansai-ben is a group of dialects that fall under the Kinki hougen (近畿方言 – Kinki dialect). There is a love-hate relationship between Kansai and Kanto speakers, the latter feeling like the former are charming, funny, obnoxious or gangster-ish, depending on the person.
Kansai-ben is spoken faster and has much more intonation. It's most noticeable feature is the change from the negative ending –nai to –hen. Sentence endings are different, with na replacing ne and de replacing da (shortening of desu). Ii (良い – good) becomes ee (ええ – pronounced "eh") and there are a whole host of unique words like okini (おおきに – thank you), honma (ほんま – really?) and akan (あかん – no good, like Kanto's dame).
To Kanto-ben and Kansai-ben speakers, Tohoku-ben (東北弁) seems like slow, country speech. It's called zuu zuu ben because of the mumbly way Tohoku speakers talk and jokes are made about how it's too cold up there to open your mouth. When Tohoku-ben speakers appear on TV, there are subtitles.
There are several phonetic differences between Tohoku-ben and Kanto-ben. The particle ga often becomes nasalized to nga, which is also not uncommon among Kanto speakers. Some stops become nasalized, such as ku becoming gu. Iku (行く – go) might sound like igu, and so on. I've heard (although never heard it myself directly) that Tohoku speakers use ora for the standard ore (俺 – I, used by men) and that it is used by women as well. Tohoku speakers also use sa relentlessly, sometimes substituting it for the particles e (へ) or ni (に).
Kyushu is the southern and westernmost island of Japan and some of its dialects are totally unintelligible to other Japanese speakers. In addition to changes in vocabulary, Kyushu-ben has some differences in how some of the vowels are pronounced. The –i at the end of adjectives changes to –ka (for example, samuka rather than samui means cold) and a whole host of different words are used such as batten instead of dakedo (だけど – but), yokka instead of so desu ne (そうですね – I see) and sukan instead of suki ja nai (すきじゃない – don't like).
In Northern Kyushu, you'll also notice the change from を to ば and from the progressive tense ～ている to ～よっと, leading to sentences such as 今なんばしよっと？ (今何をしている？) and リンゴば食べよっと (リンゴを食べている), practically incomprehensible if you are unfamiliar with the dialect.
Last but not least, the southern edge of Kyushu is home to the Kagoshima-ben group of dialects, also known as Satsugū. Although they share over three-quarters of the Standard Japanese vocabulary, the dialects are mutually unintelligible to even the neighbouring Kyushu variants, and there are many stories of natives of other regions mistaking Kagoshima-ben speakers for foreign tourists.
There are many more dialects found throughout Japan and even some tiny localities have their own distinct speech patterns. The variation makes it fun to travel around Japan but also a bit confusing.