What is the Semantic Web?

What is the semantic Web?

I seem to be asked this question on a daily basis mostly in reference to Google now using RDFA and Drupal 7 efforts to put RDFA in Drupal Core (iO1 is sponsoring ;-). The answer is not a simple one liner so I am constantly struggling to come up with a concise unifying statement that allows me to explain it to anyone. No luck yet so here's my current best effort at it.