Diesel is a fuel made from the fractional distillation of crude oil, and is used in engines and for heating.
Diesel is a mixture of hydrocarbons, in a similar way to petrol, but in diesel, the molecules are longer (C15 or more).
This gives diesel a higher boiling point, and makes it a thicker, darker-coloured liquid than petrol.
A similar - although not chemically-identical - mixture of hydrocarbons can be made from vegetable oils obtained from a variety of plant sources, if they are combined with alcohol. This is called biodiesel.