We just discussed this material's use to coat wine presses. Rashi at the end of Shabbos 20b says that it is made from (the sap of) a tree. It is not made from petroleum like the product of the same name today. Do you have any idea which plant it came from?
Moshe Perkal, Yerushalyaim
Here is what wikipedia (http://en.wikipedia.org/wiki/Tar) has to say about that:
Tar is modified pitch (resin) produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden vessels against rot. The largest user was the Royal Navy. Demand for tar declined with the advent of iron and steel ships.
Tar-like products can also be produced from other forms of organic matter such as peat. Mineral products resembling tar can be produced from fossil hydrocarbons including petroleum. Coal tar is produced from coal as a byproduct of coke production. Bitumen is a term used for natural deposits of oil "tar" - such as at the La Brea Tar Pits
(The article goes on to discuss the history of tar, which goes back to Greece and even to the iron age...)