Since it was released in May 2010, many posts and articles introducing the semantic web have embedded or linked to the film Web 3.0: A Doc, by Kate Ray. For those who are seeking a baseline understanding of web 3.0 mechanics, however, I think there are videos, presentations and web pages that do a better job of explaining the core concepts and technologies of the semantic web. Don't get me wrong: the Ray film is an interesting, informative and compellingly-presented video, and you should definitely check it out. But if your initial incursions into the world of the semantic web include the questions "how does it work?" or "how do I get me some?" then there's some really excellent resources out there that will go a long way to answering those questions in short order.
Manu Sporny's 2007 video, awash in stick figures, is for my money still the best video introduction to the semantic web for the layman. This will be six minutes and seven seconds of your life well spent.
Hatem Mahmoud's presentation starts with a history of web technologies, clearly defines some key terminology relevant to Web 3.0, and then provides a excellent walk-through of different semantic web structures. Particulary strong in describing the main microformats and showing examples of their use.
Davide Palmisano's presentation is more somewhat more technical than Mahmoud's, but this is to be expected as he delves deeper into Web 3.0 technologies. Fantastic handling of the semantic web acroynm stew, starting with a really solid explanation of RDF, followed by a brief overview of each of the core technologies related to RDF: RDFSchema, OWL, RDFa and SPARQL.
An excellent one-pager from Samantha Wong and Richard Howlett, including some links to other beginner resources.
If you're interested in the formal categorization of digital information (and you should be – really!), you'll do no better than this splendid introduction by Heather Hedden. An in-depth, but very accessible, overview of core taxonomic concepts. Controlled vocabularies, taxonomies, thesauri, controlled vocabularies, ontologies: they're all here.
Both great linked data introductions. The first, from New Tech Post, is a concise overview in Q&A format, and concludes with a brief, linked glossary. The second, by Juan Sequeda, contains a brief list of four linked data principles that is priceless – you'd also do well to check out his Introduction to: RDF.
I'll stop here: if you're new to Web 3.0 and have gone through all the resources above, you're virtually an expert by now! You'll find more beginner resources in my list of semantic web bookmarks (though be warned I don't keep on top of curating these as much as I should). If you have any other resources you'd like to recommend, please do so in the comments.