There are two primary categories of type: text and display. In general, text type is designed to be legible and readable at small sizes. This usually implies fairly clean, consistent, uncomplicated design features; more open spacing than a display face; and thin strokes that hold up at smaller sizes. Display type, on the other hand, can forgo the extreme legibility and readability needed for long blocks of text at small sizes for a stronger personality, elaborate and more expressive shapes, and a more stylish look.
Sometimes they're interchangeable, but not always. Typefaces look different depending on the size at which you view them. Spacing, proportions, and design details change optically. A text face used at large sizes can sometimes look clunky, heavy, and unattractive, and the spacing looks too open. On the other hand, display designs used at small sizes can have design features that break up, disappear, or fill in when viewed small; become less readable; and look too tight.
To avoid unwanted surprises when choosing a typeface, always try to see how it looks at the size(s) you plan on using. It's very difficult to visualize what 14-point text will look like from a 60-point showing, and vice versa. In addition, pay close attention to the spacing, and be prepared to open or close the spacing (tracking) as necessary.
