The Endless

It seems, after having abandoned his duty as one of The Endless Destruction came to truly understand what The Endless are. He says that they are simply patterns, ideas, repeating motifs and makes it clear that for their functions to be performed, no actual intervention is required on their part. In fact he goes on to say The Endless have no right to interfere with the lives of mortals.
Also, he explains his own theory about what each of The Endless represent saying that he symbolizes creation, in this same way he goes on to say that Death is to life as Despair is to hope, Desire is to hatred and Destiny is to freedom. When Dream questions what he would be in this all, Destruction says, "Reality, perhaps?" This seems to shake the very foundation of what The Endless are, as most of the others see themselves as bound to their duties while Destruction makes it very clear that without them all will go on, perhaps more sporadically, but it will continue.
Neil Gaiman has also added in the foreword to The Sandman: Endless Nights, that The Endless are not to be seen as gods, for gods need to be believed in to exist, The Endless will continue existing whether we believe in them or not, for they are The Endless.
Another thing that should be brought up is that although the first eight of The Endless may have never been human and the original seven may have never been anything but The Endless, the ninth was originally Daniel Hall, a mortal babe who was made Dream when forces outside of his control made it necessary that he take on the role, for although you can abandon your duties, there must always be one to hold the mantle of each of the seven Endless.
One thing about The Endless that seems to support the idea they were once "mortal beings" of sorts is the fact that they have been known to reproduce and bear, at least partially, human offspring.
In Sandman: Overture, which narrated the battle of Morpheus which lead him to his imprisonment, the parents of the Endless were revealed as Time , the embodiment of all time itself, father of The Endless, and Night, the embodiment of the infinite darkness that existed before the dawn of the universe and mother of The Endless.

Destiny	Death	Destruction	Desire	Despair

Delirium	Dream Morpheus Ended.	Dream Daniel Hall

The Infinite

Living Tribunal	Eternity	Galactus Galen of Taa	Mistress Death	Infinity

Elder Gods

Chton	Gaea	Set	The Wild One

Cosmic Beings

Mogo	Ego	Epoch	Love	Hate

Lord Chaos	The In-Betweener	Master Order	Kronos	The Stranger

The Celestial Toymaker	The Destroyer Sutekh

Elders of the Universe

The Elders of the Universe's origins are lost in the early history of the universe. They are some of the oldest living beings in the universe. Each of the Elders is the last survivor of their respective species, most of which evolved right after the "Big Bang", the cataclysmic event in which the universe was created.[citation needed] Their respective species were the earliest creations of the Celestials.
The Elders are virtually immortal and not only have they outlived their respective races, they have also all outlived their original galaxies. While they are not Abstract Entities, they have all attained some level of cosmic power. As the relatively few survivors of the earliest period of the universe, the Elders regard themselves figuratively as brothers. They do not often associate with each other aside from the Grandmaster's schemes.

The Challenger	The Collector Taneleer Tivan	The Contemplator Tath Ki	The Gamemaster En Dwi Gast	The Gardener Ord Zyonyz

The Possessor Kamo Tharnn	The Runner

Heralds of Galactus

The Silver Surfer Norrin Radd	Firelord Pyreus Kril	Terrax Tyros	Air-Walker Gabriel Lan	Nova Frankie Raye

War of Order & Chaos

Lords of Order

Powerful beings, they're directly responsible for upholding order within the universe and are known to empower beings to act on their stead or may even rise up to challenges towards order themselves. Some Lords of Order also have additional responsibilities.