What is semantic web?
It is easy for people to recognize the structure of a web document. Different size fonts and the layout of the document makes it easy to recognize the title (what the article is about) and any sub-headings. semantic web is a way to represent this structure so that a machine can recognize the structure of your document. When we talk about a machine reading your web page, normally we are talking about the search engine spiders. Modern search engines read and index your web pages so that they can locate your web page when some one searches the web.
Why is structure important?
If the search engine understands the structure of your document it can provide more relevant search results. They do not know if your document is about a word or if the word is simply in your document. Bing emphasizes this in their commercials. In the commercials people start rattling unrelated information about words. An example would be an article written about Albert Einstein, compared to someone stating that they are no Albert Einstein. The words are the same but the context is different. Knowing the structure of the document makes it easier to understand the context.
How do you communicate the structure of your document?
The starting point for the semantic web is using the HTML header tags H1 – H6. The text in the H1 tag is the title of your document (what your document is about). If your document has sub themes you should use increasing header tags. Your header tags should represent an outline of your document. Your document is about the words in your header tags. The content of your document is about the words in your headers, words not in your header tags are more descriptive.
This is a simplification but presents the idea. The search engines also favor word in links, links go to other document so represent important words as well. SEO is an art into itself, but here we are talking about semantic web.
What comes after the header tags?
RDF is a specification that allows you to define more detail in the structure of your document.
When you are building your web document it is important to remember that you need to provide machine readable structure in you document.