According to the Google Base site ( beta ), it is:

... a place where you can easily submit all types of online and offline content, which we'll make searchable on Google (if your content isn't online yet, we'll put it there). You can describe any item you post with attributes, which will help people find it when they do related searches. In fact, based on your items' relevance, users may find them in their results for searches on Google Product Search, Google Maps and even our main Google web search.

Key features:

From the W3.Org site

From their SEKT in a Nutshell brochure:

SEKT is a European research and development project launched in January 2004 with a lifetime of three years ...

... SEKT is developing automated techniques for extracting meaning from the Web. By generating structured descriptions of Web pages (using ontologies) SEKT is making those pages machine-processable and enabling computers to analyse information more intelligently.

Note: the article needs updating.

The Semantically-Interlinked Online Communities project ( SIOC ) has revised their web site. The content is better organized, more accessible and interactive ( using the Drupal CMS by the way ). The one-page guides provide a quick introduction to SIOC.