Hi Symo, kanji show a meaning, so nouns, verbs, adjectives are mostly written with at least one kanji. hiragana are often added to kanji in order to show grammatical things: e.g. for a verb the normal or polite form, present or past tense. katakana is used for japanized foreign words.

To better study kanji I recommend a book called "Remembering the Kanji".