Why You Should Learn Katakana (+ 40 useful words to get you started)
Katakana is mostly used for writing loan words that originate in other languages, or names and places which do not have a kanji form. This is why katakana is so useful.
Japanese characters
What are Hiragana and Katakana used for? Do I need to learn them?
When to use katakana and when to use hiragana, learn the difference.