Historically, a font is a given alphabet and its associated characters of a single size and style containing all the letters, numbers, and marks needed for typesetting. This used to be an actual box that held the pieces of lead and wood of a particular typeface. These pieces of type would be arranged on strips of metal that would determine the spacing above and below each row of type in a paragraph, called "leading". The physical weight and size of these pieces would prohibit storing all of them in one font box. Now type is held in a font folder on a computer, and it can hold all the various sizes and styles of a particular typeface, not just one. Designating a specific type inside a typeface is what is called the font.
use it in real life: Lets say you are looking at a paragraph of type in a magazine. You like the way it looks and want to know what it is. You ask "what is that typeface?" The answer is Times. Then you ask "what font is that paragraph set to?" And the answer is Times italic 12 point.
another example: You want to type an email. You go to the font folder which holds all of your typefaces of various sizes and kinds. You select Times roman 12 point. Times is the typeface, and Times roman 12 point is the font.
see also: font