Definition
Typeface vs font
A typeface is the design — Garamond. A font is one specific instance of it — Garamond Italic at 11 pt. In everyday software the two words have merged, but in print specification the distinction still does real work.
The split comes from metal type, where a typeface was the drawing and a font was a physical case of sorts at one size and style. It survives in book work because a manuscript specification names a typeface and then enumerates the fonts it requires: roman, italic, bold, small caps, and often oldstyle figures. A face that lacks a true italic or true small caps cannot do the job however good the roman looks.
Related tools & guides
See it handled in your own manuscript
Ceeryo sets your .docx as a book — justified and hyphenated text with widows, orphans and rivers controlled, drop caps, running heads — and shows you every spread before you pay anything.