In HTML there are both logical tags and physical tags. Logical tags are designed to describe (to the browser) the enclosed text's meaning. An example of a logical tag is the <strong> </strong> tag. By placing text in between these tags you are telling the browser that the text has some greater importance. By default all browsers make the text appear bold when in between the <strong> and </strong> tags, but the point to take away from this is that the strong tag implies that importance of that text. This has impact with search engines like Google who look for such tags to help figure out what the page is about.
There are many logical tags and they include:
<strong> : Strong - as above.
<em> : emphasize - usually renders (made to look like by the browsers) as italic.
<span> : a neutral inline container. - read about this distinction below.
<div> : a neutral block element. - read about this distinction below.
Logical tags, as mentioned above, have default ways in which browsers (like IE or Opera) render them. But it is understood that CSS should be used to give them their style, or in other words their 'look'.
Physical tags on the other hand provide specific instructions on how to display the text they enclose. Examples of physical tags include:
<b> : Makes the text bold.
<big> : Makes the text usually one size bigger than what's around it.
<font> : Used to apply a font-face (like Arial or Helvetica) and font-color.
<i> : Makes text italic.
Physical tags are more straightforward; <i> makes the text italic, <b> makes text bold and <font> is used to set the font face and color for the text.
So why use one or the other?
In a nutshell, physical tags were invented to add style to HTML pages because style sheets were not around, though the original intention of HTML was to not have physical tags. Why? Well physical tags are just messy, tedious, and are more trouble than they're worth (for the most part). Rather than use physical tags to style your HTML pages, you should be using style sheets (also called Cascading Style Sheets or CSS). I think someone around here wrote some articles on CSS.?