Setting Type for User Interfaces

Illustration of how saccades work
While we read, our eyes move unpredictably from word to word, rather than in a linear fashion.

Billy Whited, writing for Typekit:

By necessity, our task-oriented web applications tend to collect more isolated words and sentence fragments than their content-driven counterparts. Because of this, we cannot rely on the contextualizing effects of saccades to help improve comprehension of the textual elements of our designs, nor can we rely on the sheer presence of additional letters to assuage the unfortunate consequences of a poorly-chosen font. This makes our choice of typeface extremely important on task-driven sites.