I believe the easiest way to differentiate them is to use <acronym> if you can speak it like a word, not spell them out like a group of letters. We say “ray-dar”, not are-aay-dee-ayy-are. So we get
<acronym title="Radio Detection and Ranging">Radar</acronym>.
If spelling them out letter by letter it’s an <abbr>, like this
<abbr title="Cascading Style Sheets">CSS</abbr>.
Offcourse, Microsoft has decided not to support the
abbr element in its browsers …
Some other resources for further discussion: