Gods were any species that have incredible power, usually considered omnipotent to more primitive or less powerful beings. Often times such gods were worshipped, because of their great power, they are viewed as greater beings by mortals.
Many gods were immortal or nearly, and some used their incredible abilities to enslave beings with less power. Some gods aren't even alive at all, such as Pytho-Rex, which is an idol worshipped by a race of Venusians.
On Earth, gods are common dieties in various religions. In Christianity and Judaism, the "true God," is the creator of all things in existence. Due to his unlimited power, some primitive cultures seen the power of other physically advanced races, such as the Olympians, and they believed that they were gods as well. They were considered as such, in fact they even considered themselves gods as well. This is false however, as no power in the universe matches the true God.