eh. i'm not very good at telling the difference, i think serif is the type with itty bitty tails? in which case i greatly prefer sans serif. i just like clean fonts with nothing hanging off the ends and whatnot. verdana is my personal favorite, i use it on aim and wherever else i get font options, unless it's for a school assignment. school assignments i prefer times new roman, but only because it's standard.
i dunno, i'm picky. verdana is my go-to font, and garamond i use sometimes in decoration.