Tar is naturally found in small pits in swamp biomes. If you try to walk over it, it will drag you down under it like quicksand and will do half a heart of suffocation damage every second. It is unbreakable and it spews large particles. Sometimes it will spawn Tar Slimes.
Unless done on a flat surface it is completely inescapable once you've stepped in it and all items that touch it will despawn, like lava making it a very costly death. Tar can also be set on fire which will spread very quickly. Tar was a large cause of animal deaths in certain parts of the world 10,000 years ago.