The difference between Contextual and Physical markup. Contextual markups gives the browser the freedom to do what it believes should be done with the markup.
Different browsers do different things with the same markups.
Physical markups are more specific to what they do. Physical tags are simpler, and do a more specific thing than Contextual tags. When a tag is Deprecated it is generally shunned. The tag is still there to use, but designers urge against it, because it's function can be performed in different more pure ways. It has been deprecated in the HTML 4.0 standard.