Serifs are those little strokes at the top and bottom of most typefaces — but not the one used here. Here’s a basic picture of serifs (in red) from the Wikipedia article on the subject:
There’s more to this than that of course. The Mergenthaler Type Library categorizes its typefaces by the nature of their serifs. They classify into five groups, “Old Face”, “Transitional”, “Modern Face”, “Slab Serif”, and “Sans Serif”, which last describes the font in which you are reading these words.
Gutenberg’s earliest types were all Fraktur/Black letter (occasionally referred to as Old English), which does of course have twirly bits which could be referred to as serifs, or at least could justify the Italian printers who first introduced Roman types in their decision not to eliminate serifs. Of course, serifs, although unnamed at that time, were familiar from ancient Roman stone inscriptions, and these were an explicit model for the early typesetters. There’s some suggestion that the flicks which serifs mimic represent the action of the pen as it is released from the paper at the end of a stroke. The Roman precedent however seems satisfactorily determinative for a craft being developed in the Renaissance, where classical knowledge was being busily recovered and distributed.
was the first Roman font (though it was done as Italic only initially). You can see that the serifs are bracketed — they get thicker as they get closer to the vertical stroke of the letter. Classic old style. One might imagine Francesco Griffo who cut it for Aldus Manutius modeling his letters on Trajan’s Column:
may be taken as the typical transitional font. Though transitional serifs are little different from old style: the label seems almost to be there as a signpost rather than as anything actual. John Baskerville (1705-1776) gained a huge reputation as a printer, though his main business was japanning. The serifs on his types are bracketed still, but a little less so, and the whole letter is lighter, thinner than old style types. This was related to advances in printing technology, but doesn’t really amount to a revolution, though much excitement was aroused across Europe.
Designers exist to create difference so naturally someone thought “Why must there be brackets?” Giambattista Bodoni (1740-1813) designed the first modern face, with serifs which are just straight lines. The brackets, however, remain on the serifs at the righthand ends of horizontals: the cap E has unbracketed at the left, but bracketed serifs at the right. Aesthetics prevail over theory again!
Slab Serifs are unmistakable. Memphis can stand as an example. As you can see the serifs are aggressively straight. Such mannered fonts ought really to be restricted to display, but naturally enough we can always manage to find some innovation-mad designer who can’t resist the temptation of setting text in a slab-serif typeface.
The origin of the word “serif” is not altogether clear. The OED suggests it may have come from the Dutch schreef, meaning line, stroke or mark, though its earliest example (1785) spells the word “ceriph” which might appear to argue against that derivation. On the other hand the word is odd enough to scream “borrowed from another language” and schreef does appear to be the Dutch word for it.
Are serifs any use, or are they just decorative convention? The eye chart you are exposed to at the optician’s tends to use sans serif type, but whether sans serif or serif type is more legible seems to be difficult to establish. Legibility is affected by many factors, and little real research has been done. In so far as any conclusions can be made, it would appear that serifs are actually irrelevant to legibility. Some research seems to indicate that (surprise, surprise) we find it easier to understand the familiar than something new. So serifs are probably there because we are used to their being there.