Onomatopoeia, in Japanese called 'giongo 擬音語' is a word or a group of words that have their meaning indicated by the sounds they mimic. For example sound effects like 'bang'. Mimesis, in Japanese called 'gitaigo 擬態語' are state words like 'sparkle'. In Japanese onomatopoeia and mimesis is used a lot more than they are in western languages. It is so important and prevalant that they are separated in three categories; Giseigo擬声語, Giongo擬音語 and Gitaigo擬態語.
Are simply sounds that people and animals can make like ニャンニャン (nyan-nyan) the Japanese version of a cat's meow.
Sounds that can't be put under Giseigo. Like sounds of the nature. For example ポツポツ (potsu-potsu) the sound for rain drops. Another example is どきどき the sound of a pounding heart. Most Giongo are written in katakana but as you can see with どきどき there are a lot of exceptions.
Are used to describe an action or an emotion and they don't necessarily have to make noises. Technically these are not called onomatopoeia but fall under mimesis, because they don't try to imitate the sound but mimic the actions. For example ぺちゃくちゃ (pechakucha) means chit-chat and おどおど(odo-odo) is to feel uneasy / insecure. Most gitaigo is written in hiragana but like with giongo there are a lot of exceptions.
~ Mei mei