Metadata is data about data. For example when try to download a song from iTunes, it means not only the song, but also we will download additional information about that song (artist, track, length,etc). In this case (iTunes) that information will be stored in a format of ID3.
But metadata is not going to tell us about what should and not should to be on a profile for example. It makes everybody could put everything in their metadata even if it’s not a relevant. One argue about how metadata can make people lazy or even stupid is Corry Doctorow.
“The gathering of metadata requires people to be both the observer and documenter of their own behavior, allowing personal bias and regional variations in terminology to creep in. And there is also the problem of differing opinions on what constitutes accurate metadata; the notion that there is a “correct” way of categorizing and describing all data is fallacious.”
Despite that metadata has a negative side, the existing of this format is essential if we use that in an appropriate way. It also can be a first step for semantic web, just like what Bill Gates said when in was interviewed by Tim O’Reilly.(www.youtube.com/watch?v=Z9X-vHJ_Z-I)
There are many type of metadata, but the most popular one is Microformat. Microformats is “a set of simple, open data formats built upon existing and widely adopted standards”. Another definition provide by microformat forum is “are simple conventions for embedding semantics in HTML to enable decentralized development.”.
So what did that mean? Essentially this: marking up certain chunks of content in a standardized way that allows external applications, aggregators, and search engines to Recognize the content for what it is when crawling your website. Manipulate that content by providing access to it, collating it, or converting it into other, related formats for use in external applications and web services.
courtesy : HTML Mastery: Semantics, Standards,and Styling by Paul Haine (2006)