You can find Tar most often in large caves on the Stalactite layer that are connected to Fossil layer caves. Since all Creativerse game worlds are interveined with countless caves, exploring the underground will sooner or later lead you to a pool of Tar.
Rumour has it that if you found a body of water on the surface, you can find a body of tar under where the body of water is if you dig down. However, it has already become apparent that this is not true at all. More often, caves close to Oceans and estuaries may have Tar in them.
Like with all liquids except for Mineral Water, 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 while diving into Tar.
Tar has a base temperature of 86°F by itself.
How to obtain
Tar can be infinitely harvested as long as you won'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.
How to use
No specific recipes need to be unlocked to cook these, only the 3 (of 4 existent) 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 Sandwiches (unlocked by baking ordinary Bread in the Cooking Station).
Tar can be frozen by throwing Freeze Bombs at Tar that has been placed into the game world or can be found there in the shape of small puddles and rarely cataracts, but not by Snowcubes and also not by cooling it down with Ice, Snow or even Snow Blowers - these methods simply do not chill down Tar enough to make it freeze.
Tar is highly flammable. Currently it's the one substance that ignites the easiest and fastest in Creativerse. Maybe because it has a base temperature of 86° F by itself, even when placed into very cold biomes.
Tar starts to burn at 111° F degree (which is not exactly Fahrenheit, but more of a Creativerse-specific temperature scale).
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 please), Tar will immediately start to burn by itself, even without being set ablaze by any flame-sources nearby.
Tar will not transform into any kind of blocks by being set on fire, for example by Fire Bombs. Instead it will burn away completely, leaving nothing behind. You can also evaporate Tar by using the AOE Gauntlet Smash, but you'll need to equip a Lumite Mining Cell in order for this to work. Any weaker Power Cell will not have any effects on Tar.
However burning Tar can set any flammable blocks and plants on fire that it touches, and such it 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.
The temperature of the surrounding is one of the important factors to decide if flames can spread to adjacent flammable materials or not; except if extreme heat is being used, like emitted by Fire Bombs, Flaming Skulls or liquid Lava.
If flammable blocks start burning, the flames are then able to spread to other nearby flammable blocks, even across gaps of 1-2 blocks on worlds and player claims where the "fire spreading" option is enabled. This way a whole wooden building or forest (especially Wildwood, Parchwood and Shorewood) can burn down in the worst case.
Fire cannot be extinguished by using liquids, but it can be stopped from spreading by claiming the area and making sure that the claim (advanced) option "fire sim/spreading enabled" is disabled (by default). Fire Bombs and the like will have no effect on these claims either.
It is possible for owners of the game-world (F2P players included) to toggle the spreading of fire on a whole game-world in the basic world options ("edit world" and "disable fire spread"). Another option to stop fire from spreading would be to create forest aisles/swaths wide enough so that the flames cannot leap over.
Flames can hurt player characters with burning damage over time for about 4 seconds upon touch and can eventually kill them, the same goes for most Creatures except for Hot Feet, Warmworms and a few others (mainly event Creatures) that are immune to burning damage. Open flames are often not hot enough to transform blocks (like Sand into Salt, Hardened Lava into liquid Lava or Coal Nodes) by themselves though.
Creating Nodes from Tar
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)."
You cannot use simple Ice to cool down and transform Tar anymore like it was once possible, this option has been disabled in late fall of 2015.
Tar will also not freeze when Snow Blowers are applied, and also will not be fazed by Snowcubes and Snow Blowers, but will transform when Freeze Bombs are thrown at Tar. So it can be presumed that Freeze Bombs can create much deeper temperatures than -22°F (like Snow Blowers can in cold surroundings) at least for a split second at their impact. Again, please note that "F" might not really mean "Fahrenheit" in this game, but is a Creativerse-specific temperature scale.