The status bar style changes depending on the context. If you're in an app or an area that is presenting full-screen content/background, like photos, videos, or your home screen, then you'll have a more subdued, slightly transparent black status bar with grey/white text. If you're in a standard menu-driven app like Settings, you'll have a light grey status bar with black text. If you're in a dark, custom-themed app like iTunes U, VEVO, Notes, or Reminders, then you'll have the solid black status bar with grey/white text, but it won't be transparent (because there's no content to show behind it).
It's up to the app maker to decide what kind of status bar to show, but that's the general idea, you'll have a status bar that fits the app's theme or current context most closely so that it doesn't stick out like a sore thumb.
In rare cases, I have seen apps eschew the status bar entirely and draw their own custom clock, signal indicator, and stuff. I can't remember where I saw that, though.