microformats

Check out http://microformats.org/ to discover a set of easy to use conventions for embedding semantics in HTML. Microformats tries to add more semantics to the web we currently know. This is a pragmatic approach that contrasts with initiatives like the semantic web in it's pragmatism.

The microformats standardize ways to describe calendaring, business cards and vote-for links. For example to demonstrate how down to earth it is, the voting-for links enable you to express whether you agree or disagree with a certain link you put in your page:

some.domain.com

This example would link to the some.domain.com website and using the "rev" attribute you would express the fact that you disagree with this site. Indexing and tracking applications can then take that into account.

Check out the other microformat specifications! Or read this excellent microformats introduction.