Stolen from Wikipedia,
//Typefaces with serifs are often considered easier to read in long passages than those without. Studies on the matter are ambiguous, suggesting that most of this effect is due to the greater familiarity of serif typefaces. As a general rule, printed works such as newspapers and books almost always use serif typefaces, at least for the text body. Web sites do not have to specify a font and can simply respect the browser settings of the user. But of those web sites that do specify a font, most use modern sans serif fonts, because it is commonly believed that, in contrast to the case for printed material, sans serif fonts are easier than serif fonts to read on the low-resolution computer screen.//