Presentation ;)
Limit your font choices to two or three for your entire site. Reference is always made by typographers to the horrific mistakes made early on when word-processors came into mainstream use a few years ago. Magazines and other publications were printed with dozens of fonts used on every page, just because the designers could. This led to the pages looking like a big mess. You should use very few fonts to avoid the same fate.
Stick with a sans-serif font for your main content, as text in this family of fonts is easier to read from a computer screen than serif fonts. Keep your main body text between sizes 2 and 3 (10 and 12 point). Also remember that you should use the common fonts that everyone will have on their computer (found in the font face tutorial), so they see the same page you do.