As far as the human realm goes, the world of Supernatural functions just like our real world. This means all of the same religions exist are practiced in the same way as in the real world. However deities from all religions are, by rule, beings that are real in some form or another.
The world is created by God, and by all accounts that God is the same one all Abrahamic religions worship. God (and Darkness) is the most powerful, nearly omnipotent and omniscient, beings in existence. God can be prayed to, and if he is present he might even listen. Angels can also be prayed to, but only if they are addressed by name.
Other deities, which were created by God in ancient times, are powerful but fall significantly short of his power or even that of archangels.
Those deities, better known as pagan gods or more accurately, The Old Gods – are a race of immortal supernatural entities, with each being a prime authority in the religion or pantheon that they established or belonged to. Despite having a role in the human ancient religions and possessing free will, the plurality of them are cruel, petty, uncaring, and only concerned with themselves and their survival. (Source: Fandom Wiki)
Demigods are the offspring of deities and humans (or other creatures). These hybrid deities can be ones identified in lore, but there can also be ones that are to this day unidentified and unheard of.
The power of the old gods resided in their worship and sacrifices made to them. They were powered by magic forces, and the lack of worship in modern days has caused many of them to lose power or even forget who they really are. All deities can be killed in some way.
It’s possible for an ignorant human to accidentally summon or make a deal with a deity. A lot of the lore that can be used for such a thing is widely available, but usually dismissed as humbug. Ancient rites and magic spells performed in pagan traditions are real, and if one stumbles upon the correct version and performs it, it will probably work.