What Exactly Is "Metadata"?
Let's step back and talk about metadata. What is metadata? Why is it important? Broadly speaking, metadata refers to any "data about data." This is a pretty nebulous definition by any standard. I think of metadata as being separated into two broad categories:
System metadata consists of things like what my book reviewer was thinking about: number of characters in a field of a database record, datatypes, etc.
Content metadata is less structured and often of a more qualitative nature: things such as author, copyright information, keywords, create date.
Broadly speaking again, system metadata is about system properties (such as describing the characteristics of a database table or of an XML schema), while content metadata refers to specific pieces of resources: a particular image, a particular article, a particular web page. RDF is focused on content metadata.
"So what?" you may say. "I've got XML and that's perfectly good for representing content metadata. XML lets me invent my own tags, ergo I can do perfectly well without your fancy RDF." You might say the same about XML itself, thoughwhy use XML when you can use your own proprietary coding scheme? RDF is useful for the same reasons that XML is useful: interoperability, vendor support, and all the inherent benefits of using an industry-standard approach. Using RDF syntax to represent metadata means that any metadata consumer that can interpret RDF will be able to recognize and parse the metadata.
But RDF is about more than syntax. RDF is a "model" for metadataa standard way of thinking about metadata that transcends any particular coding scheme. It's this model that makes RDF so potentially powerful.
The RDF model is an analog for how people really store and process information. As a child, you learn about relationships by example and by reference. You know what a car is because you have a myriad of examples of cars stored in your brain, which all add up to a composite idea of "car." When someone tells you "I crashed my car yesterday," you know exactly what they're talking about because you know what a car is and you understand "yesterday" and the concept of possession. The RDF model works the same way, except that the car is any URI and your brain is the Internet.