Toujin No Negoto　★　唐人の寝言

Japanese has a strange type of word that are not found in English, that have an interesting form. Called 擬態語 gitaigo and 擬音語 giongo, these words are usually made up of four syllables, with the first two repeated. My explanation sucks, but you'll get it soon. These words are not usually written in kanji.

Gitaigo are words are used as actions, adverbs, nouns, or adjectives. I guess the best way to show you is with examples. The gitaigo is in bold:

★ だらだらするな！ daradara suruna- Don't be lazy!
★ あの子のりのりとダンスしてるだね ano ko norinori to dansu shiterun da ne- That girl's really dancing in high spirits (as an adjective).
★ ますます好きになってます masumasu suki ni nattemasu- I'm coming to like you more and more (adverb).
★ だんだん好きになってきた dandan suki ni natte kita- I gradually came to like you (as an adverb).
★ ばらばらになった barabara ni natta - It fell apart (as an adjective).
★ うるせぇ、もじゃもじゃ urusee, mojamoja- You're really annoying, shaggy-hair (used as an insulting nickname).
★ 涙で顔がくしゃくしゃになった - Her face went gloomy with tears (used as an adjective).

On the other hand, giongo have the same repeated syllables, but express instead sound words (or onematopeia). For example 'bang bang,' 'knock knock,' 'thump thump.' In this way, sometimes they can be used as verbs aswell. Because they are onematopeia, you will often find them written in katakana, especially in manga.

★ あの人に近づいたらドキドキします ano hito ni chikazuitara dokidoki shimasu - when I get close to that person, I get nervous.
★　ドンドンとノックした dondon to nokku shita - He knocked with a bang bang.

I'm not even going to try and touch on how many possible combinations there are. If you ever go drinking with a Japanese person, shoot some random syllable combinations at them (e.g. Does guchagucha mean anything? How about taratara? or biribiri?). You'll be surprised and annoyed at how many there are.

Gitaigo and giongo are actually my weakest section in the JLPT, so maybe others also have trouble with this phenomenon. For you guys, check out this site, which has a lot fo various sound effects (not limited to giongo).

If you want a more detailed and technical explanation, along with some other examples, check out theWikipedia page on it.

There is a gitaigo/giongo dictionary online here.