Diesel or diesel fuel in general is any fuel used in diesel engines. The most common is a specific fractional distillate of petroleum fuel oil. There are other types of diesel such as biodiesel,or gas to liquid diesel, but petrodiesel is the most common form of diesel fuel.
Diesel is much more popular in Europe than in the United States as a fuel source. In Europe 35% of passenger cars are powered by diesel, that could rise to as high as 50% over the next 10 years.
That is in part because American consumers, spoiled by cheap gas prices and hampered by enduring negative perceptions, have been down on diesels. But that is all changing with rising gas prices and the desire for cleaner diesel and government regulations forcing diesel to be cleaner.