Katakana is often refered to as an alphabet. Each character represents a sound. Hiragana is similar to katakana, but it is used for different reasons.
Katakana is used for foreign words, places & names. Katakana is also used for words that represent sounds (ding-dong, boom-bang). Technical and scientific terms such as the names of animals, plants and minerals are also often written in katakana.
Katakana can be used on signs, buildings, advertisements and products. It's also used to add emphasis. The sign above is in Katakana, if you can't read it yet don't worry it doesn't take long to learn katakana.
When a word written in Katakana has a double vowel a choonpu （－） is used to indicate it. Words with a double 'i' are written with the second 'i'.
A small 'tsu' character called a sokuon （ッ） indicates a double consonant. The sokuon is place before the kana that is being doubled. It can't be used to double a 'n'. The Katakana for 'n' will be added before to make it a double 'n'
A Nakaguro （・） is a 'middle dot' used often in foreign words where the reader might not know where the space is between two words. In Japanese there are no spaces between words.