Is <th> only semantically different from <td>?

Question

Their looks are the same actually

Answer 1

No, generally unformatted text in a th table header element will be bold. But you should use CSS to style the way you want them to appear. If you use something like Yahoo Reset, then they will just be a semantic difference. To avoid cross-browser styling difference I recommend using something like Yahoo Reset which removes all browser default styling from most elements.

Answer 2

Since there is a semantic difference most browsers also bold and center the contents of a <th> tag.
This is irrelevant if you are using a reset stylesheet, but there is a presentational difference by default.