Three character sets are used when writing the Japanese language. They are called:

Hiragana
Katakana
Kanji

The hiragana and katakana character sets are phonetic syllabaries. They are used to spell out words. This is much like the Roman alphabet can be used to spell out words in the English language.

Each character in hiragana and katakana has no meaning, it just represents a sound.

Of the phonetic syllabaries, hiragana is more rounded and katakana is more angular.

Kanji is a very large set of characters that were adapted from ancient Chinese characters. With kanji, each character has both meaning and sound. Most kanji have more than one meaning and more than one sound.

Chinese characters (kanji) were first used in Japan during the 5th century. Around the 9th century, Japan developed its own form of writing based on phonetics and this resulted in the hiragana and katakana syllabaries.

Theoretically, the entire Japanese language can be written in hiragana or in katakana since all sounds of the language are included in each of those syllabaries. In practice though, all three character sets are used.

In Japanese newspapers, magazines, and books, you will see a mixture of hiragana, katakana, and kanji. You may occasionally also see letters of the Roman alphabet, most typically for writing foreign acronyms such as USA.

Kanji is usually used when writing nouns, verbs, and adjectives. However, some words are native to Japan and there is no way to write them using Chinese characters. In such a case, the word is usually written in hiragana.

Hiragana is used in three main ways:

to write grammatical endings for verbs and adjectives. For example, hiragana endings are used to modify the root word for "open" so that it has meanings such as "open", "opened", "does not open" and "did not open". to place grammatical markers in sentences. They are words (or in some cases a single character) which indicate the relationship of the preceding word or phrase to the rest of the sentence. For example, the particle wa follows the topic of the sentence and the particle o follows the direct object of the verb. to write Japanese words for which there are no kanji.

Katakana is usually used to write words borrowed from other countries. Katakana can also be used any time the writer wants to bring attention to what is being written. This is similar to using italics.