Stripes on suits are always vertical, but come in a number of different styles.
The first, foremost, and most classic is the pinstripe. A pinstriped suit, particularly a navy pinstriped suit, is an extremely traditional look, almost a uniform of sorts for the businessman. Pinstripes, as the name suggests, are very narrow though generally prominent stripes, most often in white, although gray is a not-uncommon choice, especially on dark gray or black suits.
These, and all stripes, help to make the wearer appear taller, as they draw the face upwards, and can make a heavyset man appear thinner, by breaking up and drawing attention away from the solid midsection. This is particularly true for thin, close set stripes like the pinstripe.
There are also other stripes, including the thicker chalk stripe, as an option for the gentleman's wardrobe. These stripes, which are generally set farther apart due to their greater width, are significantly less formal than the pinstripe, and while continuing to provide the appearance of increased height, are less appropriate for large men, as they can draw attention to ones girth.