Source: http://en.wikipedia.org/wiki/Synthetic_oil
For detail, refer to the website.
Synthetic oil is oil consisting of chemical compounds which were not originally present in crude oil (petroleum) but were artificially made (synthesized) from other compounds. Synthetic oil could be made to be a substitute for petroleum or specially made to be a substitute for a lubricant oil such as conventional (or mineral) motor oil refined from petroleum. When a synthetic oil or synthetic fuel
is made as a substitute for petroleum, it is generally produced because
of a shortage of petroleum or because petroleum is too expensive. When
synthetic oil is made as a substitute for lubricant refined from
petroleum, it is generally to provide superior mechanical and chemical
properties than those found in traditional mineral oils.