Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
Microdata is a method for associating metadata with a specific element in HTML markup, via the "class" attribute. This is typically best done using standardized prefixes on the attribute values to avoid what are known as "collisions" (one term being used in two different senses). An important development in this area is the emergence of schema.org, an initiative championed by Google and endorsed by the major web browsers, which provides a sort of "registry" of standard terms and their prefixes that browsers are supposed to understand. For example, there are terms for dates and times and calendars, for "friend-of-a-friend" (FOAF) associations, for recipes, and a host of others. For educational content, there's a new schema.org format called LRMI from the Learning Resources Metadata Initiative that provides an essential pedagogical and educational vocabulary. Such microdata standards save you the trouble of having to come up with terms of your own, and ensure you're using terms in the same way lots of other publishers do and in a way that browsers and browser-based technologies can properly handle.
Another more complex but more powerful method is Resource Description Framework (RDF), usually expressed in XML as RDFa ("RDF in attributes"). This is a core technology of the Semantic Web, and it is already widely used in the library world. It's also at the heart of many advanced search and semantic technologies. Its simple subject-predicate-object structure provides a way to describe the relationship between one thing and another, which ultimately creates a "network" of relationships. Not only does this enable extremely powerful semantic associations, it also enables inference: when you know "Liza Daly [subject] works for [predicate] Safari [object]" and "Andrew Savikas works for Safari," you can infer "Liza Daly and Andrew Savikas are colleagues" without explicitly connecting those dots. Semantic metadata is not something publishers commonly use today, but it is very powerful and likely to become increasingly important.