True God is a title granted to those beings who have at least half of a special bloodline flowing in their veins. People believed that this bloodline have a heavenly origin, so they are destined to be deities upon birth. True Gods are not humans nor part of the contemporary races, they are transcendent existences that have many unimaginable characteristics and properties.
True Gods are stronger than Godkings. If an Immortal Emperor used his strongest blow, then maybe he would be able to kill a True God. Other cultivators could not kill a True God even if they used an Immortal Emperor True Treasure.
At least in some cases True God's cultivation was totally different from any other races. In the case of Calamity God his power came from unlocking the Eighteen Shackles that restricted his race original power.