One term that I only just learned relatively recently is 都道府県 (とどうふけん), which sounds like a Street Fighter move but refers to Japan's 47 prefectures. I guess when I was actually in Japan I didn't really watch much TV and newspapers were a little too intimidating, so I didn't often encounter words like this.
But at any rate, why is there a special word for the prefectures collectively? Why not just 47県? Well, because they don't all share the same governmental status or structure. Wikipedia breaks it down pretty well:
"The prefectures of Japan are the country's 47 first-order subnational jurisdictions on a state or provincial level: one "metropolis" (都 to?), Tokyo; one "circuit"/territory (道 dō?), Hokkaido; two urban prefectures (府 fu?), Osaka and Kyoto; and 43 other prefectures (県 ken?). Prefectures are governmental bodies larger than cities, towns, and villages."
So while we just call them all prefectures in English, I think in Japanese there is some distinction.