A proportional font uses varying widths to display each of its letters and symbols, while a non-proportional font uses the same fixed width to display its characters.
Some of the most popular proportional fonts are Times New Roman, Arial, Verdana and Georgia. Most people find these types of fonts easier to read and more aesthetically pleasing than non proportional fonts, which is why they appear more frequently in published material.
Non-proportional fonts are mainly used for formatting purposes. When characters need to be lined up neatly in fixed columns, a non-proportional font such as Courier or Monaco can be used. Old fashioned typewriters and early computers used fixed-width fonts exclusively.