A font is a specific typeface of a certain size and style. For example, one font may be Arial 12 pt bold, while another font may be Times New Roman 14 pt italic.
Most word processing programs have a Font menu that allows you to choose the typeface, size, and style of the text. In order to use a font, you must have it installed on your computer. Windows provides access to fonts using the Fonts control panel. The Mac OS stores fonts in a Fonts folder and includes a separate "Font Book" application for managing fonts.