"Serif type is easier to the eyes than sans serif."
There's an old principle in the graphics world that goes "Serif type is easier to read because the serifs draws your eye from character to character." Hence, sans serif type is oftentimes used for headings and short quantities of text.
Truth to tell, all fonts can be made readable (except, well, maybe for Wingdings) with the ideal design. With sans serif, although it needs more leading than serif type, it can give your documents a very modern look, and is the popular body text in Europe.
This rule make sense especially since most textbook are written in serif typefaces. But, the majority of websites use sans serif in body text. So my main concern is should sans serif mostly be used for headlines because of clarity purposes? I find that sans serif as just as easy to read in comparison to serif typefaces. Of course with proper leading they can flow just as easy as serift typefaces. Variations in thickness and strokes of letters at least to me sometimes make serifs typefaces the ideal for body text. I I purposely made this paragraph in serif Verdana to illustrate another example as our text mentions as well with respects to serif typefaces.