[CrimsonNataku]

Katakana is used only to spell out foreign words.
i.e. ルレストラン
Resutoran
Restuarant

Hiragana is used to spell everything else (all Japanese words). Katakana can, however, be used to emphasize a word that's normally in Hiragana (it would be the equivalent to an italics in English). Kanji is used in writing so that the meaning of the sentence in hiragana isn't mistaken for something completely different since a lot of words look the same.
i.e. かえる
Kaeru
Frog, or: to go home (verb)
When speaking, you can tell the difference because of the intonation of the word (The "frog" kaeru is spoken with a rising intonation, the "to go home" kaeru has a falling intonation). However, in writing, there is nothing to tell you what intonation is being used, and thus there is kanji.

蛙
Kaeru
Frog

帰る
Kaeru
To go home

Does this help?