One of three writing systems used in Japan, hiragana is a phoneme-based kana (alphabet) consisting of 46 characters, of which many can be modified with the use of one of two accidental characters (a small circle, maru, or a double tick, ten ten, both in the upper right side of the character). It is the first form of writing taught in Japan, shortly followed by katakana and later, kanji. Hiragana is typically more cursive in appearance, and is used as the 'general purpose' kana set -- that is, it could be used exclusively, and in place of both kanji and katakana.
The original hiragana set consisted of 51 characters, but several were dropped over the years due to similarities in sound with other characters. The yi and ye sounds were dropped in favour of the i and e sounds respectively. Likewise, the wi, wu and we sounds were dropped in preference to their easier to pronounce base vowel forms.
The kana character which occupies the wo position is unique in that it is pronounced o, (no 'w' in the sound), and that it is never found in a word -- it stands alone as a determinant.