Web fonts and typography
Up until a few years ago, choosing one of the two or three available typefaces, specifying a colour, and setting a font size was all you could do to affect the typography of a website. While web typography is still very limited when compared to that of print, the CSS support in modern browsers now allows web designers to actually think about typography.
Knowing how type on the web works, and more specifically how type on a computer screen works, will help you understand how it is different from print typography. And that will help you make better typographical design choices. I won't go into further details, and instead point you to Andy Hume's comprehensive SitePoint article on the subject: The Anatomy of Web Fonts.
The article explains the technology used to display type on computer screens, goes through the basic principles of web typography, including comparing some common web fonts, takes a look at how you can work around the current limitations, and hints at what may be available in the future. A great read.