From Envirowiki

Oil is a group of substances that are hydrophobic (repel water), and are liquid at room temperature. In terms of the environment, "oil" usually refers to petrochemical oils, but it can also refer to vegetable oils and animal fats, and essential oils.

Oils are usually combustible, and are used as fuels. They are also used as lubricants, and some animal and vegetable based oils have nutritional value as food sources.