Katakana is one of the four basic alphabets in use in Japan, the others being Hiragana, Kanji and the Roman alphabet. The useful aspect for foreigners though is that Katakana is used predominantly for western words and where as you may know how to pronounce something in Hiragana or Kanji you may not necessarily know what it means.
For example, if you can read the word "ice cream" in Katakana, you know what you're ordering but even if you can read "bonenkai" in Hiragana, you may well still be at a loss to know you've just been invited to a Christmas party. And this can lead to problems.
Back in the day I worked at adidas in Japan (and yes, it is spelt with a lower case "a" though whoever thought of this had clearly never used a auto-spell checker or would have given it up as a lost cause). We were sourcing product from various countries across Asia when we received a message from a factory in Thailand. They had shipped 400,000 T-shirts with a slight typo in the Japanese tags. Instead of correctly printing:
which is how to spell "adidas Japan", they had misprinted it forgetting the modifiers and mistaking the "n". The result was we had 400,000 T-shirts on their way with this: