The World-Wide Web, like many hypertext systems, is based upon a markup language , in which authors annotate text to describe the texts role or appearance. There are two different views of markup, logical markup and physical markup. HTML has some logical tags and some physical tags. In logical markup tags describe the role a piece of text plays.
Physical markup describes the appearance of a piece of text.
When designing a web page, it is important to know what kind of markup you are using, and why you have chosen to use that type of markup. I generally advocate logical markup, with a little bit of physical markup when appropriate to enhance readability. There are a number of reasons to support logical markup. Some benefits accure to the author of a logically-marked text, others to the readers. Logical markup also empowers readers.