A recent Computerworld article from March 13, 2008 puts it like this:

Yahoo said that its support of standards such as microformats and RDF, or the Resource Description Framework, are aimed providing users with better search results by improving the understanding of content and the relationships among content.

[...]

Yahoo also announced that it will launch a beta tool to let third parties add data to Yahoo Search results within several weeks. For example, a restaurant could use this tool to add reviews or other data to Yahoo Search results for queries about the eatery. Developers can build enhanced results applications by accessing structured data that Yahoo will make available through public application programming interfaces and in its index. The structured data is available to Web site owners through feeds or the supported Semantic Web standards, Yahoo said.

Michael Arrington, a blogger at “TechCrunch,” wrote that Yahoo’s announcement means “we can expect the Web to get itself organized in a hurry. At stake is a significant amount of traffic from Yahoo search, and anyone that may choose to build applications on top of this data.”

In addition, Yahoo’s support for Semantic Web standards like RDF and microformats is the incentive Web sites need to adopt them, Arrington said.

“Instead of semantic silos scattered across the Web …Yahoo will be pulling all the semantic information together when available, as a search engine should,” he added. “Until now, there were few applications that demanded properly structured data from third parties. That changes today.”

This example demonstrates how to use the Yahoo! YUI Editor and OpenCalais API as Zend Framework Service to flavour your content with some semantic.