I have to admit, when I first heard about this I thought it was kind of crazy and impossible to think information could really be captured via html markup and other types of semantic writing. If you want to learn about the broader idea of the semantic web you can read an article posted by smashing magazine.
Apparently Google is going to start investing in this idea, at least in buying a company that is investing in this idea
Google just purchased Metaweb. Now that Google acquired Metaweb, they have really bought access to an impressive database of web entities. What are entities? Metaweb calls entities a word that is a person, place or thing. One entity can have many different types of names or different names referring to the same entity. For example, you could refer to my current employer in a number of ways: PTC, Parametric Technology Corporation, Parametric Technology, etc. What metaweb does is realize that everyone is talking about the same thing! They take this ever expanding database and open it up to the world with access to capture and access the entities.
Why does that matter? You will have to read into it a bit more but it is searching not just words like Google but the meta information which gives context to the words. This helps in numerous different ways. It allows for more direct SEO, searching, ability to pull in information from other places, etc.