What is the difference between <strong>, <b> tags and <em>, <i> tags?

The <strong> and <b> tags are used to denote text that should be shown in bold. The <em> and <i> tags are used to denote text that should be shown in italic. The main difference between the two sets of tags is that <strong> and <em> carry semantic weight, meaning that they indicate the importance or emphasis of the text, whereas <b> and <i> are purely presentational and do not convey any particular meaning. So if you want to emphasize something for its importance, use <strong> or <em>, otherwise use <b> or <i>.