What is the <Strong> tag for?

In HTML, this makes any text contained between it and it's end tag </strong> into bold text.

Same as BOLD

It's usually for bold text, but the browser has some discretion how to render the text. It's the same relationship that exists between <EM> and <I>; see this question:
http://answers.yahoo.com/question/index;...

It highlights the text in a browser specific way.

For instance, on a browser on your cell phone, it may make more sense to make to italicize or underline the text to make it stand out. In a PC or Mac based browser, the site can set the CSS to make "strong" stand out any way the designer wishes.

The guys who originally invented HTML, intentionally didn't want to provide a lot of specific formatting. The idea being that terms like h1, h2 and strong, would be defined by the browser.

Later on, designers started demanding more control for PC and Mac browsers. Thus the table and font tag were born, and eventually the advent of CSS.

The originators of HTML were right. Now that you can make a page with 14 point Verdana with a hanging indent, you can't see it on most non-pc device like a cell phone.

The moral of the story is this: If you think your web page will only be used by Mac and PC users, you can use all the formatting that CSS allows. If you want it to work on any device, use the old school tags like "strong".