Denim is graded in terms of weight per square yard of fabric, in three categories: light, medium and heavy. The material usually weighs from 5 oz. to 20 oz., although exceptions of extremes like 30 oz. do exist. Most jeans are made of 12 or 14 oz. denim. Lighter denim is mostly used on skirts, shirts and other garments.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary