In traditional typography, a font is a particular size, weight and style of a typeface. Each font was a matched set of metal type, one piece (called a "sort") for each glyph, and a typeface comprised a range of fonts that shared an overall design.
In modern usage, with the advent of digital typography, "font" is frequently synonymous with "typeface". In particular, the use of "vector" or "outline" fonts means that different sizes of a typeface can be dynamically generated from one design. Each style may still be in a separate "font file"—for instance, the typeface "Bulmer" may include the fonts "Bulmer roman", "Bulmer italic", "Bulmer bold" and "Bulmer extended"—but the term "font" might be applied either to one of these alone or to the whole typeface.