For understandable reasons, people tend to get very confused by the new technologies used to add metadata to HTML5 pages: should you use microdata, microformats, metatags or RDFa? Right now, the best answer is “all four”, but that usually just adds to the confusion.
With a few caveats and reservations, it is possible to categorize the best use of each format:
Traditionally, phone numbers have been placed on web pages as a simple string of digits, indistinguishable from any other content. When mobile users visit sites, they have an expectation that phone numbers should be useful, leading to direct contact.
We achieve that with two approaches: adding a link with a tel protocol, and inserting microdata. There are also a few considerations we must make for desktop applications, most particularly Skype and Internet Explorer.
In HTML most tags are semantic: that is, they carry meaning, but there is no way to provide meaning for every piece of content. This impoverishes the web: machine-readable tags that provide context make for a richer, more semantic internet. But short of an XML utopia, there's no way to cover every possibility.
Instead, there are several measures that add to, or extend, the semantics of web pages:
