Tar is found in the Overworld and in The Nether. It is a black liquid that doesn't flow. Tar acts like lava, it burns and its a liquid. It also lights surrounding flammable block on fire. Unlike lava, items that fall in tar will not burn. If you mix it with lava or water you can get obsidian. When found, it is surrounded by stone blocks, even in The Nether. It can be picked up and placed with a bucket.
A bug with this is it can spawn on top of the nether. It has been recorded that you can gain entry to the void above the nether by swimming through a pool that spawns this way.