How are kanji, hiragana, and katakana used?

Many nouns are written in kanji, verbs and adjectives are written mostly in kanji-hiragana combination, particles and auxiliary verbs in hiragana, and loan words in katakana.
For example, in the sentence "Watashi wa doubutsuen de panda o mita" (I saw a panda at the zoo), watashi (I = noun), doubutsuen (zoo = noun), mi of mita (part of the verb 'saw' ) are usually written in kanji.
Sentences written entirely in hiragana and katakana can be understood, but since there are thousands of homonyms in Japanese, it was decided that kanji, which are ideographic, should be included for clarification purposes.