"God" is a term used to describe any immensely powerful immortal being, that embodies some kind of concept or idea.
There are only six 'true' Gods in existence as the others were all byproducts of The Goddess of Creation. These six were the creatures that naturally formed from the remains of the First God after the creation of the Multiverse.
A God is made of pure energy and cannot be killed. If they are mortally wounded, their physical form will disperse and reconstitute at a later time. However, the energy can be absorbed by others given the right circumstances. This happened forcefully with The Dragon Gods and The Lich in an alternate timeline.
Gods can become more powerful from the amount of people who worship them, or believe in the ideas they embody. The God of Death bypasses this by using the energy of Souls to maintain his position as the strongest of the Gods.
They can seemingly change their size and appearance at will, as seen by The Dragon God, The Goddess of Creation and The God of Time.