When you think of HTML style you probably instantly think of CSS (Cascading Style Sheets), but style with straight HTML really refers to the formatting of the characters on the screen using HTML tags to do it.
There are two types of style tags within HTML:
tag the information according to its meaning (i.e. header, code, etc.)
tag the exact appearance of the information
Many of the styles in HTML documents can be achieved using either type of style tags. For example, the physical style tag <i> makes text italic and in most browsers the logical style tag <em> makes text italic.
Why Do We Need Two Types?
HTML was developed from SGML (another markup language) and in SGML, the presentation (style) is divorced from the content. For example, when you were in school, you were probably taught to write essays by first writing an outline. Then you could write the document and put in the formatting. HTML, in its pure form, wants to focus only on the content of the document, and not how it looks. It leaves the look and feel to something else (the browser, CSS, etc.).
Logical Style Tags
Used to define a word or phrase within a document. Typically displays in italics.
Used to note titles of books and movies etc. Typically displays in italics.
Used to indicate computer code. Typically displays in a fixed-width font.
Used to emphasize a word or phrase. Typically displays in italics.
header (1 - 6)
Six different levels of header. Each level displays differently.
Used to indicate something to be typed in. Typically displays in a fixed-width font.
Used to indicate a sampling of literal characters.
Typically displays in bold.
Used to display computer programming variable information. Typically displays in italics.
Physical Style Tags
change the font face, size, or color
fixed-width (typewriter) font
Which Type Should I Use?
Both types are supported in HTML 3.2, and while some of the physical styles (<font> specifically) are deprecated in HTML 4.0, the browsers still support them.
However... HTML is moving away from defining the look and feel of Web pages, leaving that to CSS. If your readers are mostly using 4.0 browser, then you should seriously consider using logical tags with style sheets to define how they look. If you do that, your pages will be compatible with future browsers that might not support physical styles (because they are not in the HTML specification).
Learn more about these and other tags in the HTML Tag Library.