Short answers, pulled from the story.
The word deity derives from a Proto-Indo-European root meaning to shine. This linguistic thread connects the ancient Indian Deva, the Greek Zeus, and the Latin Deus to a shared human impulse to worship the light.
The transition from vague prehistoric engravings to the systematized gods of the Egyptian state occurred in the later part of the 3rd millennium B.C.E. This shift marked a move from fluid spiritual beliefs to rigid theological structures that mirrored the political power of the Pharaohs.
Egyptologist James P. Allen estimates that more than 1,400 deities are named in Egyptian texts. Christian Leitz suggests the number reaches thousands upon thousands.
Unlike the omnipotent God of later monotheistic traditions, the deities of ancient Greece were distinctly human in their flaws and limitations. These gods could be injured in certain circumstances, and their power was not absolute.
The principal group of gods, the Aesir, and the fertility-focused Vanir, went to war before eventually reconciling through a truce that involved exchanging hostages and intermarrying. This mythological event blurred the lines between the two groups.
In Buddhism, devas are numerous but still mortal, living in a heavenly realm before dying and being reborn like all other beings. This state is achieved through leading an ethical life and accumulating good karma.