RDFa is an extension to HTML5 that helps you markup things like People, Places, Events, Recipes and Reviews. Search Engines and Web Services use this markup to generate better search listings and give you better visibility on the Web, so that people can find your website more easily.
Linked Data is a relatively new way of thinking about how we publish data. At present, there is a great deal of data on the Web, but it is either locked down in a private database, or published in a way that is not very useful to computers. The data we have today is disconnected and lives in data silos. These data silos are not linked to one another, which goes against the core design of the Web. It is important that we fix both the data silo-ing and data linking problem. If we do so, computers will be able to use the data more easily and thus make our lives better as a result. RDFa is a way of publishing Linked Data in HTML5 documents — it solves both the data silo-ing and data linking problem.
There are immediate benefits for integrating RDFa into your website today. By using RDFa to express Google Rich Snippets or schema.org, your web pages will be displayed in an enhanced format on all major search engines. You can also use Facebook's Open Graph Protocol, which is based on RDFa, to express concepts that are contained in your web pages, like people, places, events, movies and recipes. The search and social companies are serious about indexing RDFa content, which means that you may want to get serious about adding it into your pages before your competitors do.
RDFa not only works in HTML5, but is a general technology for Linked Data markup in a variety of HTML-like languages. This means that it works well in XML documents, SVG documents, as well as HTML4, XHTML1 and XHTML5 documents. In fact, if your markup language is based on XML, then you can already use RDFa in your documents today. There is a version of RDFa for beginners called RDFa Lite. The full version of RDFa is more expressive and allows Web authors to solve just about any structured data markup problem that they might throw at it. Whatever popular Web document format you use, chances are that RDFa already works in it.
There are a number of good reads available on this website covering everything from basic RDFa usage, to markup recipes, to advanced technical concepts.
There are a variety of tools available for RDFa, ranging from online structured data preview tools, to validators, to processing tools and software libraries.
If you are a software developer, there are a number of tools and libraries that you can use in your software projects to extract and process RDFa markup in Web documents.