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. Many browsers permit readers to customize the way logically marked text is displayed. You may have encountered a simpler version in the font and color settings of Netscape. By using logical markup, you give your readers the option to describe how their text is displayed. Logical markup can make it possible to distinguish between similar looking pieces of text with diverse meanings.