title: Knowing what RDFA vocabulary to use
body: <p>How do we know which vocabulary/namespace to use to describe data with RDFa?</p>
  
  <p>I have seen a lot of examples that use <code>xmlns:dcterms="http://purl.org/dc/terms/"</code> or <code>xmlns:sioc="http://rdfs.org/sioc/ns#"</code> then there is <a href="http://www.youtube.com/watch?v=ldl0m-5zLz4" rel="noreferrer">this video</a> that uses FOAF vocabulary. </p>
  
  <p>This is all pretty confusing and I am not sure what these vocabularies mean or what is best to use for the data I am describing. Is there some trick I am missing?</p>