There’s absolutely no way that I’ll be able to do more than scratch the surface of this convoluted and fascinating subject. So let’s get scratching…
The basic idea of typography stems from the original typefaces of movable type printing, but the flexibility and DIY nature of the digital age have transformed the field into something between a design topic and an obsession for most people involved.
The theories underlying typography are microcosms of the theories of design itself; readability, uniqueness, and space play essential parts.
Readability seems self-evident, but the ‘absolute readability’ of the font itself is only the beginning; things like proper contrast and sufficient size can make even somewhat convoluted typefaces perfectly acceptable on the page. Nothing is worse than bad color choices for a page, but bad font colors (think small pink text on an orange background) are either a mark of incompetence or (at best) a lame black hat SEO strategy.
Space is pretty obvious too, but as I said before there are “micro” as well as “macro” considerations. The space between lines, between letters, between the various tiny little black strokes that make up the letters themselves…all of these contribute to the effect. Whether you’re creating type or simply choose between fonts, it only makes sense to see how the font will work with itself and on the intended design.
An interesting option, most often either overused or underused, is the variations of a single font (i.e., italic, bold, point size, color) to delineate varying emphasis and existing options. Likewise, using more than one font has a tremendous potential for overall crappiness but when subtly deployed can be a great way to create hierarchy. Titles, headings, and captions are regularly displayed with varying point size, but may benefit from other ways to establish hierarchy as well. And I’ve seen several menu structures that forgo the usual graphical boxes for a nicely chosen set of subtle type variations.