We get this question from time to time so I thought I’d share an excerpt from Print Industry’s June newsletter:
“Serif vs. Sans Serif” Typefaces and Legibility
Serifs are the little strokes on the ends of letterforms. You will see them on such typefaces as Times, Baskerville, Garamond, and Palatino. The strokes, or tails, lead your eye from one letter to the next. Various studies have proven over the years that, with ink-on-paper, a serif typeface is easier to read than a sans serif face (sans serif typefaces might include Helvetica, Avant Garde, Futura, and Franklin Gothic).
Keeping in mind the ease of reading type printed on paper in serif typefaces, the exact opposite is true on a computer monitor. The regular grid of pixels in strict horizontal and vertical alignment apparently minimizes the legibility of serif type. So choosing a sans serif typeface such as Helvetica (or Arial) would be prudent in this case.”