Resource Description Framework (RDF and RDFa) is the term applied to the organization of data prepared specifically for the search. The standards set by RDF/RDFa enable the communication of granular, specific information, embedded within web pages and xml feeds to be extracted via search.
The terms "microformats" refers to the specific tagging protocols that can be used within html or xml that specify the searchable pieces. These tags specify a class that is used to communicate this information directly to the engine. This enables us to embed product information, like brand, category, description, price, etc. within the context of an html page, using existing css instructions like class. Similar standards apply to xml feeds.
One of the most important applications can be the use of these formats within web commerce, where traffic is directed to pages fulfilling a search imperative. And the document can be anything - a review, an article, a product page, an affiliate comparison, etc. The point being that html can now be used to contribute knowledge to the searcher via the microformat tags.
And the use of these tags can be easily brought onto the page via automation, since this already is supplying details as data points. So converting existing documents to microformat specs is not even difficult. And the impact could be huge - someone reading the review of your product could easily find any specific detail, like the price, or special purpose, or brand, or color, etc. because the information is flagged and easily recognized by the tags.
For web commerce, this is a huge leap forward if this becomes universally accepted. it means that the information most people make decisions on, are always available on any site using microformats. Google is currently experimenting with it - rather seriously, if you consider they've even built tools to let you test your rich snippets (snippets that have microtags), to see how your site may appear in their search.
It's worth experimenting with this right now, because if/when this goes mainstream, the early adopters will have the lead. We're seeing evidence that it already has traction, especially in getting information from product reviews and product descriptions.
Microformats in product info
http://www.google.com/support/webmasters/bin/answer.py?answer=146750
Microformats in reviews
http://www.google.com/support/webmasters/bin/answer.py?answer=146645
More on rich snippets
http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=99170
Rich snippets testing tool
http://www.google.com/webmasters/tools/richsnippets
Background on Microformats
http://microformats.org/about/
Technicals
http://www.w3.org/TR/xhtml-rdfa-primer/
Sign up form: Let Google know you're using rich snippets
http://www.google.com/support/webmasters/bin/request.py?contact_type=rich_snippets_feedback
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