Here's a pretty good explanation from The Complete Manual of Typography, by James Felici: "No two words in typography are as commonly misused as font and typef

Here's a pretty good explanation from The Complete Manual of Typography, by James Felici: "No two words in typography are as commonly misused as font and typeface. A typeface is a collection of characters—letters, numbers, symbols, punctuation marks, etc.—that are designed to work together like the parts of a coordinated outfit. A typeface is an alphabet with a certain design. A font, in contrast, is a physical thing, the description of a typeface—in computer code, photographic film, or metal—used to image the type. The font is the cookie cutter, and the typeface is the cookie (see Figure 3 1).† When you look at a page of type, you can say, “What typeface is that? ” or “What font was used to set that? ” But you can’t say, “What font is that? ” because you’re not looking at a font; you’re looking at the product of a font. The confusion between the terms arises largely from the ambiguous use of the term font in computer programs, most of which have a Font menu. Although that menu lists what fonts are available for use by the program, it could just as easily be called the Typeface menu, as it also lists the typefaces available for your pages. In fact, since some fonts contain data for more than one typeface, it would be more accurate to call it the Typeface menu." † Figure 3.1 shows an example font (the computer code) and its corresponding typeface.