Japanese writing systems
Katakana is used when writing down foreign names, places, and words of foreign origin (If you are not from Japan, your name can be written in katakana). Without knowing katakana, one might find it almost impossible to order fast food in Japan, or read billboards on the street. This is because Japan uses so many words borrowed from other languages (click here for more on borrowed words).
The structure of katakana is similar to hiragana (if you know one the other is easy to learn). The main difference between the two is katakana is composed of straight lines, where hiragana uses curved lines.