Characteristics and usage of hiragana
The hiragana syllabary consists of 48 syllables and is mainly used to write word endings, known as okurigana in Japanese. Hiragana are also widely used in materials for children, textbooks, animation and comic books, to write Japanese words which are not normally written with kanji, such as adverbs and some nouns and adjectives, or for words whose kanji are obscure or obselete.

Hiragana are also sometimes written above or along side kanji to indicate pronunciation, especially if the pronunication is obscure or non-standard. Hiragana used in this way are known as furigana or ruby. In horizontal texts, the furigana appear above the kanji and in vertical texts, the furigana appear on the right of the kanji. In newspapers it is a legal requirement for furigana to be attached to kanji which are not included in the official list of the 1,945 most frequently-used kanji. Newspapers in fact rarely use kanji not included in this list.

Characteristics and usage of katakana
The katakana syllabary consists of 48 syllables and was originally considered "men's writing". Since the 20th century, katakana have been used mainly to write non-Chinese loan words, onomatopoeic words, foreign names, in telegrams and for emphasis (the equivalent of bold, italic or upper case text in English). Before the 20th century all foreign loanwords were written with kanji.

Katakana are also used to write Ainu, a language spoken on the northern Japanese island of Hokkaido.

Source: http://www.omniglot.com/writing/japanese.htm