Tar can be infinitely harvested as long as you don't pick up the initial source-"block", but only it's liquid extensions. Since Tar is viscous and flows / extends quite slowly it is recommended to let it flow directly downwards to "multiply" it. By this new Tar "sources" will be created one below another and will continue to flow downwards slowly.
When swimming in Tar, a drowning scale will show up, and if filled, player characters will lose health while staying in this liquid until they have drowned (if not healed up constantly or getting out before being "defeated" by the Tar...). Also the vision / screen is darkened significantly when diving into Tar.
Tar can be used in the Cooking Station to cook Tar Soup, Tar Bread and Tar Sandwiches - all of which are the fastest burning types of Fuel for the Forge as of now (June 2016), but can also be consumed. No specific recipes need to be unlocked to cook these, only the basic recipes for Soup (unlocked together with the Cooking Station), Bread (unlocked by harvesting Wheat that is grown on tilled land from Wheat Seeds) and/or Sandwiches. Also other types of Food can be made from Tar as an alternative Liquid.
Tar is highly flammable. Currently it's the substance that ignites the easiest and fastest in Creativerse.
If placed in hot environments like the Lava layer or even at hot places in Jungles or Savannahs (mind the temperature displayed in the top right corner), Tar will immediately start to burn by itself, even without being set ablaze by any flame-sources nearby.
However burning Tar can also set any flammable blocks and plants on fire that it touches, and can cause vicious spreading Forest Fires in warm to hot biomes, causing a whole forest or wooden house to burn down to nothingness in time. Currently (as of June 2016) you can stop spreading fires only by creating forest aisles/swathes by taking away blocks - at least 2-3 blocks wide, otherwise fire can jump over narrow gaps unto other flammable blocks.
Tar is otherwise used to cook Tar Bread, Tar Sandwiches and Tar Soup at a Cooking Station. Tar Bread and Tar Soup are currently the best kind of Fuel for the Forge available in Creativerse. There are no special crafting recipes to unlock these kinds of food - the Cooking Station only offers 4 recipes (unlocked by crafting the Cooking Station, by harvesting Wheat, by cooking common Soup and common Bread) and will provide different kinds of food depending on which ingredients you will use.
And here a quote about that from developer Thor from Playful:
" ... Tar->Coal->Diamond is always a topic of discussion that comes up over and over again. In fact in the last release we made tar far more viscous (It now only flows one block out from the source horizontally), incredibly difficult to see in while swimming, and super flammable (don't put torches next to your coal generator)."