According to HowStuffWorks, traditional diesel oil, also referred to as petrodiesel, contains crude oil that has been extracted from geologic formations beneath the Earth's surface and is then treated through a fractional distillation process at a plant. This process results in a thick, oily, liquid combination of hydrocarbon chains that has a very high boiling point.
HowStuffWorks notes that diesel fuel is easier to refine and requires fewer steps to filter than regular gasoline, which is why diesel fuel used to be cheaper. Since around 2004, the value of diesel fuel has increased as a result of growth in the industrial and construction sectors of the United States and China. Additionally, diesel fuel is more scarce because oil refineries in the Gulf of Mexico have been shut down. The price of diesel fuel usually increases in colder winter months as demand for heating oils increases.
The process used to refine heating oils is quite similar to that of diesel fuel. Traditional petrodiesel contains an unwanted amount of sulfur. Ultra-low-sulfur diesel is considered to be less toxic and produces far fewer noxious emissions. Wikipedia states that since 2006 North America, Europe and the United Kingdom have predominantly switched from petrodiesel to ULSD.