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Google has updated many of their structured data developer docs yesterday. Aaron Bradley spotted it and posted about the changes in detail on Google+.
In short, they the introduction section only slightly in terms of content. They updated this page to redefine what structured data markup is with a revised definition. Plus updated several other sections. Aaron covered them in detail on Google+, so check that out.
Here is a before and after screen shot from Aaron:
Forum discussion at Google+.
