Font
A font is one style of letters, like a plain round style or a classic style with small feet.
In a simple example
Your site uses one font for titles and another for paragraphs.
Why it matters to you
The font sets the mood of your words, from friendly to serious, before anyone reads them.
Do not confuse with
Typeface and font are used for the same thing in daily talk. Both mean the style of the letters.