Typography
Letterforms treated as a visual system, balancing legibility, rhythm, and expressive form.
In practice
Typography is the craft of arranging type so text is legible, readable and expressive. It involves choosing typefaces, sizes, weights, spacing and line lengths, and setting them in a clear order. Typography decides whether a medicine leaflet is easy to follow and whether a concert poster feels loud or refined. Good typography often goes unnoticed; readers become aware of it mainly when it fails.
Often confused with font
A font is a file or set of letters; typography is the practice of choosing and arranging such letters on a page or screen.
Try this
Set the same short paragraph in a serif and a sans serif typeface and note how its tone changes.