The Japanese written language is composed of three writing systems: kanji, hiragana, and katakana.
Kanji are the characters borrowed from Chinese. These number in the tens of thousands, but to read Japanese you really only to need around 2,000 kanji.
Hiragana and katakana are both phonetic systems, each numbering 46 basic symbols, or letters. Each hiragana symbol has an exact corresponding katakana symbol and vice versa.
This series covers the basic hiragana and katakana symbols in addition to all the modified forms (those with diacritic marks and the combined forms) that make up the standard possible sound spectrum for the Japanese language, for a total of 104 items.