I see katakana as a tool to emphasize the word in question. Because it has this capability, it is often used to represent words from foreign languages (many of which have kanji or hiragana terms that convey similar ideas) and in onomatopoeia.
Wikipedia mentions that katakana is also regularly used for scientific terms and the names of plants, animals, and corporations.
Ben Bullock, author of the sci.lang.japan website, mentions a few other uses. He explains that katakana was the first Japanese language used on computers, and, as a remnant of that, all banking information is written with katakana. Additionally, he tells us that katakana marks foreign accents and, interestingly, was used until the 1940s where we now use hiragana in marking okurigana.
I'll leave you today with my friend's clever understanding. Takumi Murayama, born in Japan and a native speaker, has a new, insightful take: "I always thought of Katakana as what you would use if you didn't know what to use," he responded.
Examples of katakana:
Format:
(Usage – かたかな – Translation – Location of word)
Emphasis – マツザカ – Matsuzaka (Name) – Red Sox baseball jersey
Loan word/emphasis – ラーメン – Ramen – Japanese menu
Onomatopoeia – ガーン – Expression of shock – Hetalia (Manga)