Katakana is the syllabary for foreign words, foreign names, country names... Eg: Television (テレビ) , Nura (ヌラ), Spain (スペイン). Hiragana is the syllabary of Japanese words. Every Kanji lecture can be expressed using Hiragana but remember that Kanji is the correct way to write Japanese. Eg: Kanji 日 means "Day" Kanji 日 is readed ひ (Hiragana hi). Hiragana ひ means nothing.

maybe ..., Hiragana is used in several cases. For example, if the noun is 「Japan」, the kanji is 日本, the hiragana is にほん 　　　If the verb is 「get up」，the kanji and hiragana is 起きる, the hiragana is おきる 　　　If the preposition 「from 」．the hiragana is ~から． Katakana is often used as a loanword.

Most Japanese can be expressed using hiragana. I use katakana for names of people and places. In everyday conversations, like facebook, etc, hiragana can be used exclusively. Some people prefer to use kanji because it shortens the sentences, and therefore the time to read.