It's better to use san-serif than serif fonts on web, as their simpler letter form remain readable at low resolution. Serif fonts need more pixels to display their extra details. I was wondering why I seldom see any san-serif fonts for body text on print design?
I don't deal in print, but I have read quite a bit about fonts in the past.
Recent studies have shown that serif vs. sans serif on a computer display is not really what affects readability, even at lower resolutions. Print, however, is a different matter. The studies consistently indicate that in print, serif based fonts are easier to read. That said, some studies I have seen have said that even in print, it doesn't really matter. It may be a cultural artifact more than anything.
There are several other factors with fonts that affect readability more than serif/sans serif, such as x-height, kerning, leading, ascender and descender heights. Maybe it's just that the typical sans serif fonts aren't great in these other areas as well when it comes to print.
Here's a fairly in-depth study I found that has some nice info to explain some of these things: http://alexpoole.info/which-are-more-legible-serif-or-sans-serif-typefaces
Let me preface my answer by saying the evidence is all over the place on this topic. It seems prudent to suggest that choice of typeface has a relatively minor effect on reading speed or legibility given our current understanding.
Here are some sources that seem to suggest that serif text has a statistically significant effect on readers (either through improvements to legibility, recall or reader preference):
Certainly the conventional wisdom (and what was taught to me at design school) is that serif text tends to be easier to read. In general the research consensus seems to point to no significant difference in legibility or speed of reading on account of serifs alone:
It's important to note that academic research into readability is generally performed by measuring reading time and/or comprehension; factors that require long-form text to be meaningful. In short-form use such as signage, this is the best research I'm aware of into typeface legibility (and you can see that they're testing things like reading from a distance, word recognition and efficiency).
There's a huge distinction between body copy (paragraphs of running text) and headings and other larger type. Sans serif fonts (in their modern form; text without serifs exists for thousands of years in engravings and hundreds of years in hand-lettering—see the street labels) were produced for larger, bolder uses such as signage and headings (the first commercially-available sans serif font was designed for two-line/very large uses). It wasn't until 1832 (positively recent in the history of Western typography) that a commercially-available lowercase sans serif face (referred to then and now as "Grotesque" type) became available. In those larger uses there's no real suggestion—conventional wisdom or otherwise—that serif text is in any way better for that purpose (including in print).
In general "readability" comes down to semiotics: the ability for the brain to convert the curves and splotches on the page into letters and words that carry meaning. Hypothetically, the familiarity of a given typeface should reduce the time (and effort) it takes to consume it (that's why logotypes are so easy to process—you become so used to seeing those letters in that font in that order that you don't really even need to read them to process the whole word or phrase as having the meaning that it does). In practice, research has demonstrated that familiar typefaces—ones readers have been exposed to many times before—can be read more quickly:
In a classic case of a chicken-and-egg problem, the fact that we've generally chosen serif fonts for long form text has meant typographers have generally designed them for long form text. You rarely see a serif face with an x-height as low as Futura or tiny counters like in some black and ultra bold sans serif faces, for instance. They almost always have real italics (as opposed to obliques), include lowercase numerals far more often than sans serifs and often have more flexible ligatures and other modern typographical features.
That's not to say a book typeset in Helvetica can't look beautiful compared to similarly typeset Garamond. There are plentiful tricks in the designer's arsenal to even the playing field and improve the legibility of sans serif (such as opening the leading and reducing the line length, as well as choosing less geometric sans faces that include features like stress and distinctive forms).
Long story short: your font choice matters more for reasons of branding and design than for any other reason.
My understanding has always been that serif fonts form a subtle line across the page at the bottom of each letter form which makes it easier for your eye to follow line by line for large chunks of copy.
I think a lot of UX designers (myself included) prefer sans-serif because they seem simpler and more economical. Considering a lot of web content is for short, tight blurbs of copy rather than long articles, serif fonts are less useful in a web environment anyway, although longer articles probably still benefit.
(Another option is giving wider white space between lines to also create an easy-to-follow line.)