Unlike most of the typefaces used on screens, which were designed for print and intended to be read on paper, Verdana was designed for use on the computer screen, created from the beginning to be easily readable at small sizes, with simple curves and large, open letterforms. Because the forms are so open, the counters (the negative spaces in the letters) do not fill in even when bolded, enhancing the legibility of the heavier version of the typeface. Moreover, the letterforms are spaced more widely than in a print font so they are legible even when displayed in computer applications that don't control spacing, and letterforms that look alike (such as i, I, and 1), are designed to be as dissimilar as possible. In addition, certain letters are spaced so that they never touch, regardless of combination (an f next to an i, for example), because at small sizes connecting letters can form illegible blobs.
from Standard Deviations, 2011