Creosote is a yellowish, poisonous, oily liquid obtained from the distillation of coal tar. (Distillation is the process in which the components of a substance are separated by boiling and subsequent condensation.) Coal tar is a by-product of the process of "dry" distillation, or carbonization, of coal into the solid fuel product coke.
Crude creosote oil, also called dead oil or pitch oil, is used as a wood preservative. Railroad ties, poles, fence posts, marine pilings, and lumber for outdoor use are all soaked in creosote. This treatment can greatly extend the life of wood that is exposed to the weather.