In her blog's footer, Alice declares her content to be freely reusable, as long as she receives due credit when her articles are cited. The HTML includes a link to an appropriate Creative Commons license:

A human clearly understands this sentence, in particular the meaning of the link with respect to the current document: it indicates the document's license, the conditions under which the page's contents are distributed. Unfortunately, when Bob visits Alice's blog, his browser sees only a plain link that could just as well point to one of Alice's friends or to her resume. For Bob's browser to understand that this link actually points to the document's licensing terms, Alice needs to add some flavor, some indication of what kind of link this is.

She can add this flavor using the rel HTML attribute (which we'll write as @rel so as not to repeat the word "attribute" too often), which defines the relationship between the current page and the linked page. The value of the attribute is license, a HTML keyword reserved for just this purpose:

With this small update, Bob's browser will now understand that this link has a flavor: it indicates the blog's license.

Alice is happy that adding HTML flavor lets Bob find the copyright license on her work quite easily. But what about the article title and author name? Here, instead of marking up a link, Alice wants to augment existing text within the page. The title is an HTML headline, and her name a sub-headline:

To indicate that h2 represents the title of the page, and h3 the author, Alice uses @property, an attribute introduced by RDFa for the specific purpose of marking up existing text in an HTML page.

Why use dc:creator and dc:title, instead of simply creator and title? As it turns out, HTML does not have reserved keywords for those two concepts. Alice could boldly choose to write property="title", but how does a program reading this know whether "title" here refers to the title of a work, a job title, or the deed of a piece of real-estate property? And, if every web publisher laid claim to their own short keywords, the world of available properties would become quite messy, a bit like saving every file on a computer's desktop without any directory structure to organize them.

To enforce a modicum of organization, RDFa does not recognize property="title". Instead, Alice must indicate a directory somewhere on the web, using simply a URL, from where to import the specific creator and title concepts she means to express. Fortunately, the Dublin Core community has already defined a vocabulary of universally useful concepts, including both creator and title, where title indeed means the title of a work. So, Alice:

1. imports the Dublin Core vocabulary using xmlns:dc="http://purl.org/dc/elements/1.1/", which associates the prefix dc with the URL http://purl.org/dc/elements/1.1/, and
2. uses dc:creator and dc:title. These are short-hands for the full URLs http://purl.org/dc/elements/1.1/creator, and http://purl.org/dc/elements/1.1/title.

In RDFa, all property names are, in fact, URLs.

Alice's blog contains, of course, multiple entries. Sometimes, Alice's sister Eve guest blogs, too. The front page of the blog lists the 10 most recent entries, each with its own title, author, and introductory paragraph. How, then, should Alice mark up the title of each of these entries individually even though they all appear within the same HTML page? RDFa provides @about, a new attribute for specifying the exact URL to which the contained RDFa markup applies:

We can represent this, once again, as a diagram connecting URLs to properties:

Alice can use the same technique to give her friend Bob proper credit when she posts one of his photos:

The trouble with Bob

The trouble with Bob is that he takes much better photos than I do:

about="http://example.com/bob/photos/sunset.jpg" property="dc:title"Beautiful Sunset by property="dc:creator"Bob.

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Notice how the innermost @about value, http://example.com/bob/photos/sunset.jpg, "overrides" the outer value /posts/trouble_with_bob for all HTML inside the div with the corresponding @about. And, once again, as a diagram that abstractly represents the underlying data of this new portion of markup:

Alice already has contact information displayed on her blog.

The Dublin Core vocabulary does not provide property names for describing contact information, but the Friend-of-a-Friend vocabulary does. In RDFa, it is common and easy to combine different vocabularies in a single page. Alice imports the FOAF vocabulary and declares a foaf:Person. For this purpose, Alice uses @typeof, an RDFa attribute that is specifically meant to declare a new data item with a certain type:

Then, Alice can indicate which content on the page represents her full name, email address, and phone number:

Note how Alice didn't specify @about like she did when adding blog entry metadata. What is she associating these properties with, then? In fact, the @typeof on the enclosing div implicitly sets the subject of the properties marked up within that div. The name, email address, and phone number are associated with a new node of type foaf:Person. This node has no URL to identify it, so it is called a blank node.

Next, Alice wants to add information about her friends, including at least their names and homepages. Her plain HTML is:

First, Alice indicates that all of these friends are of type foaf:Person.

Beyond declaring the type of data we're dealing with, each @typeof creates a new blank node with its own distinct properties, all without having to provide URL identifiers. Thus, Alice can easily indicate each friend's homepage:

And, of course, each friend's name:

Using @property, Alice specifies that the linked text ("Bob", "Eve", and "Manu") are, in fact, her friends' names. With @rel, she indicates that the clickable links are her friends' homepages. Alice is ecstatic that, with so little additional markup, she's able to fully reuse her existing, human-readable data for the machine-readable version.

Alice is tired of repeatedly entering information about her friends in each new social networking sites. With RDFa, she can indicate her friendships on her own web page, and let social networking applications read it automatically. So far, Alice has listed three individuals but has not specified her relationship with them; they might her friends, or they might be her favorite 17th century poets. To indicate that she, in fact, knows them, she uses the FOAF property foaf:knows:

Using rel="foaf:knows" once is enough to connect Bob, Eve, and Manu to Alice. This is achieved thanks to the RDFa concept of chaining: because the top-level @rel is without a corresponding @href, it connects to any contained node, in this case the three nodes defined by @typeof. (The @about="#me" is a FOAF/RDF convention: the URL that represents the person Alice is http://example.com/alice#me. It should not be confused with Alice's homepage, http://example.com/alice. You are what you eat, but you are far more than just your homepage.)