Hiragana and katakana are 2 phonetic alphabets. Hiragana is used for Japanese words and particles, katakana is most often used for words and names of foreign origin. Another phonetic way to write Japanese is romaji. There are 46 characters.

hiragana: あ　い　う　え　お　さ　し　す　せ　そ

katakana: ア　イ　ウ　エ　オ　サ　シ　ス　セ　ソ

romaji: a i u e o sa shi su se so

Kanji are characters of chinese origin. These make up the bulk of Japanese writing. Kanji typically have more than one way which they can be read. Kanji can be used alone or in compounds with other kanji. There are 2 main types of readings for Kanji characters: The "kunyomi" and the "onyomi".

For example: 水　(water) kunyomi = みず　　onyomi=スイ

水　（みず）　water (by itself with kunyomi)
水泳　（すいえい）　swimming (onyomi in a compound)
水虫　（みずむし）　athletes foot (kunyomi in a compound)

a word can also be a mix of kanji and hiragana. For example:

食べます　（たべます）　to eat