Typography rules and terms
Typography is, quite simply, the art and technique of arranging type. It's central to the work and skills of a designer and is about much more than making the words legible.
Choosing a font
Your choice of typeface and how you make it work with your layout, grid, colour scheme, design theme and so on will make the difference between a good, bad and great design.
Hierarchy and scale
If all type was the same size, then it would be difficult to know which was the most important information on the page.