A font is the overall design for a set of characters. It describes the size, weight, and spacing of a character and shouldn’t be confused with a typeface, which is a more general term.
Courier is a typeface; Courier 24-point bold is a font. Computers display fonts in either a bitmap or a vector format. In a bitmapped font, each character is represented by an arrangement of dots. In a vector font system, the shape or outline of each character is defined geometrically. Since a vector font is scalable according to the defined outline, a vector system can make many differently sized fonts from one defined set of characters. Currently, the most widely used vector font systems are PostScript and TrueType.