Vegetable oils are derived from plants and are composed of triglycerides. Nominally, oils are liquid at room temperature. Although many different parts of plants may yield oil, in commercial practice oil is extracted primarily from the seeds of oilseed plants.

The temperature-based distinction between oils and fats is imprecise, since definitions of room temperature vary, and typically any one substance has a melting range instead of a single melting point.

The "modern" way of processing vegetable oil is by chemical extraction, using solvent extracts, which produces higher yields and is quicker and less expensive. The most common solvent is petroleum-derived hexane. This technique is used for most of the "newer" industrial oils such as soybean and corn oils.

Another way is physical extraction, which does not use solvent extracts. It is made the "traditional" way using several different types of mechanical extraction. This method is typically used to produce the more traditional oils (e.g., olive), and it is preferred by most "health-food" customers. Expeller-pressed extraction is one type, and there are two other types that are both oil presses: the screw press and the ram press. Oil seed presses are commonly used in developing countries, among people for whom other extraction methods would be prohibitively expensive. Nut oils are generally used in cooking, for their flavour. They are also quite costly, because of the difficulty of extracting the oil.