A banner ad measuring 120 pixels wide and 240 pixels tall.
According to the IAB, vertical banner specifically refers to a banner ad measuring 120×240. Technically, there are other banner ad sizes that are also vertical in nature. Skyscrapers ads, for instance, are significantly taller than wide, but they have their own specific name.
Vertical banners are often found in the margins on the left or right side of a Web page. In fixed sponsorships, they are sometimes “stacked” vertically to create a combined effect.
see also: Online Advertising Tools