To get the bold text we will use the <b> tag.
<b>This text is bold </b>
Generally, the bold text is used to emphasize certain words or certain terms in within a sentence.
<p><b>Bold</b> text is useful to emphasize certain <b>terms</b></p>
Bold text is useful to emphasize certain terms
It can also be used to write a definition. But, we'd better exemplify:
<p><b>Negrita</b> - corresponding word in Spanish for bold</p>
Negrita - corresponding word in Spanish for bold
The HTML strong tag is meant to go a little bit further. It normally display a bold text on a desktop computer, but it has a different behavior on PDA for example. On some PDA all text is bold. So you can not bold a bold text, but the strong tag it just underline the text to make it stand out. On the other hand on some speaking programs for blind people, the strong just lowers the tone so that the person listening understand the importance of the highlighted part.
<p>This a <b>bold word</b> and this <strong>other word</strong> will have a similar appearance.</p>
This a bold word and this other word will have a similar appearance.
So just to be clear, bold - give style to an html element, while strong is a semantic tag, that stands out the keyword in a different way depending of the context.