Once you're in the Google app, just click the camera to activate Goggles. From there, you can identify and search for info on landmarks, logos, books, DVDs, etc. And as Google continues to tweak the function—it's still in Google Labs—you'll eventually be able to identify animals, foods, and whatever else Mountain View's mad scientists cook up.
It's not clear yet if the iPhone version has have the same augmented reality powers of its Android cousin, but we'll know as soon as it rolls out.
Speaking of which: because you need a camera with autofocus, Google Goggles on iPhone only works on the iPhone 4 and 3GS. It's currently only available in English, and will be rolling out around the world starting today. Which is why if you need me in the next couple of hours, I'll be here refreshing the App Store. [Google Mobile App via Google Mobile]