HTML5 : RDFa and Microdata for search engines

The web is a rich, distributed repository of interconnected information organized primarily for human consumption. On a typical web page, an XHTML author might specify a headline, then a smaller sub-headline, a block of italicized text, a few paragraphs of average-size text, and, finally, a few single-word links. Web browsers will follow these presentation instructions faithfully. However, only the human mind understands that the headline is, in fact, the blog post title, the sub-headline indicates the author, the italicized text is the article's publication date, and the single-word links are categorization labels. The gap between what programs and humans understand is large.

MicroData Formats
HTML5 Microdata: Welcome to the Machine

Microdata is a subset of making a document have meaning to machines, just as it has meaning to a reader of the document. With meaning, I mean providing the meta data in a way that can be used by a machine reading the document and to allow that data to be processed


RDFa

RDFa, which provides a set of XHTML attributes to augment visual data with machine-readable hints. We show how to express simple and more complex datasets using RDFa, and in particular how to turn the existing human-visible text and links into machine-readable data without repeating content