lucy24 - 9:03 am on Sep 27, 2011 (gmt 0)
Official answer: <strong> is semantic-- well, sort of-- while <b> is unequivocally presentational. You can define either one in your CSS to look like whatever you want it to look like.
<b> is bold. <strong> is what the browser decides it should be. Right now it happens to be bold, as <em> happens to be italic. At least in English.