Typography is the art and techniques of type design and arranging type. In traditional typography, text is composed to create a readable, coherent, and visually satisfying whole that works invisibly, without the awareness of the reader. Even distribution with a minimum of distractions and anomalies are aimed at producing clarity and transparency. The arrangement of type is the selection of typefaces, point size, line length, leading (line spacing) and letter spacing.
Readability and legibility are also often confused. Readability is most often and more properly used to describe the ease with which written language is read and understood – it concerns the difficulty of the language itself, not its appearance. In contrast, legibility describes how easily or comfortably a typeset text can be read. It is not connected with content or language, but rather with the size and appearance of the printed or displayed text.
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http://www.thethinkingblog.com/2007/08/how-to-mix-fonts-typeface-cheat-sheet.html