An Earth with superheroes and supervillains existing in it. Sentient alien species (such as Kryptonians and Thanagarians) and even functioning interstellar societies are generally known to exist, and the arrival of alien spacecraft is not uncommon. Technologies, such as artificial intelligence and faster-than-light travel, are functional and reproducible, though they are highly experimental and difficult to achieve. Demonstrable magic exists and can be learned.
Prime Earth was formed by the temporal merging of the disparate DC, Vertigo and WildStorm universes, in an effort by Pandora and The Flash to restore the Timestream to its intended nature.
In this newly made universe, costumed crime fighters are a relatively new phenomenon, with Superman being the first super-powered hero to come to public knowledge. Since then, several other heroes have come to the fore, with the formation of the Justice League. The success of the Justice League activities inspired the United Nations to create the government-sponsored team Justice League International.
Of course, there are also the supervillains, individuals that use their powers to commit evil, as well as N.O.W.H.E.R.E., an organization with a nefarious interest in superhumans and their abilities.
Many of the superhumans on Earth owe their powers to the "metagene", a genetic feature of unknown origin, which causes some people to develop superpowers when exposed to dangerous substances and forces. Others owe their powers to magic, genetic manipulation (or mutation) or bionics (see below). A large power gap resides between most superheroes and civilians. Still others owe their powers to not being human at all (see races, below). There are also superheroes and supervillains who possess no superhuman powers at all (for example Batman, Robin or Green Arrow), but rival their effectiveness with specialized equipment or "to the absolute limit of human potential" training in special skills, such as martial arts.
The humans first began using costumed identities to fight or commit crime during the 1930s. The first superheroes included characters like the Crimson Avenger and The Sandman. In November 1940, the first superhero team, The Justice Society of America, was formed. During World War II, all of America's heroes were banded together as the All-Star Squadron to protect the United States from the Axis powers. However, due to a magical spell cast by Adolf Hitler (using the Spear of Destiny and the Holy Grail) the most powerful heroes were unable to enter Axis-held territories, leaving the war to be fought mainly by normal humans such as Sgt. Rock. After the war, under pressure from the paranoid Committee on Un-American Activities, the JSA disbanded. While many types of heroes were active afterwards (mainly non-costumed, such as the Challengers of the Unknown or Detective Chimp), it wasn't until Superman's public debut that a new generation of costumed heroes became active. Soon after, the Justice League was formed, and they've remained Earth's preeminent superhero team; most DC heroes (such as the Teen Titans) have either belonged to the League at some point, or have connections to it.
As a general rule, being a superhero does not require powers anywhere near omnipotence. Furthermore, even major heroes and cosmic entities have distinct vulnerabilities, such as: Superman's weaknesses to magic, kryptonite, and red sun light; Green Lantern's initial problems with wood or the color yellow (which have since been largely overcome); or Batman's lack of superhuman powers, which he supplements with keen intellect, constant training, and specialized technology.
Superheroes are generally accepted or even praised—Superman and The Flash actually having museums dedicated to them—by the general public, though some individuals have decided that "the metahumans" must be dealt with less passively. Thus, an organization called "The Dome" was formed to help superheroes who needed to fight crime across international borders; the superhero group called the Global Guardians were their main agents. However the Dome eventually lost out, as its United Nations backing went to the more famous Justice League.
The American government has had a more wary approach, however. Back during World War II they started "Project M" to create experimental soldiers to fight in the war, such as the Creature Commandos. Most of these experiments remain a secret to the public. Currently, the government deals with metahumans and similar beings through its Department of Extranormal Operations (DEO). Covertly, they use an organization of costumed (but non-superhuman) agents known as "Checkmate". The government also formed Task Force X (known as the "Suicide Squad") for "black ops". Most members have been captured supervillains (and thus expendable), and were strongly "encouraged" to join (often with offerings of clemency if they survive their extremely dangerous missions).
Outcast personalities are often relegated to the world of DCU supervillainry. They are then usually well versed in heists, kidnappings and robberies. Villains with meek powers contrive schemes of extraordinary complexity, yet—because of their simple talents—they only call the attention of powerless superheroes like Batman, or lesser superheroes like Booster Gold. When caught, any prison sufficient enough to contain these villains is suitable. More powerful villains strive to contest for greater goals like world domination and/or universal acclaim (from the public and their villainous peers). Usually more powerful enemies are imprisoned in maximum level facilities—such as Belle Reve Penitentiary (which also was secretly Task Force X's headquarters) and even alternate dimensions or outer space—because they cannot simply be killed by a bullet, electricity, or poison.
Supervillains sometimes also form their own groups, but these tend to be short-lived because most villains simply do not trust each other. Most such teams are formed by a charismatic and/or fearsome criminal mastermind for specific purposes; an example is the Secret Society of Super Villains of which there have been several versions. Most villain teams are usually small, having been formed of individuals who know each other personally, such as the Central City Rogues, or have some other reason to work together (mercenary groups like the H.I.V.E., fanatical cults such as Kobra, etc.).
There are a few intelligent races living on Earth that the public at large did not know about until recent times.
the last survivors of Atlantis, who changed themselves into water-breathing forms, including the human-like Poseidonians and the mermaid-like Tritonians.
brief test subjects of Project Cadmus who fled to the Underworld below Metropolis.
a tribe of highly intelligent, telepathic gorillas living in an invisible city hidden in Africa; this is the home of Gorilla Grodd.
There are many intelligent extraterrestrial races as well. Curiously, a large number of them are humanoid, even human-like, in form (such as Kryptonians, who outwardly appear identical to Earth-born humans); some can even interbreed with Terrans. Some of these races have natural superpowers, but they're usually the same for all individuals of the same race, unlike Earth's metahumans. This was explained by the fact that in Earth's distant past Martians experimented on humanity, severely culling the metahuman potential; this means that a species that was meant to have a wide range of powers, like Tamaranians or Kryptonians, ended up "just...human". However, there are also plenty of nonhumanoid races as well.
The DC Universe has had many natural and cosmic disasters happen to their alien civilizations. The Martians were destroyed by war, the Kryptonians by a dying planet, and the Czarnians by plague. Even the Almeracian Empire was victim to impending destruction by Imperiex.
There are several lesser beings in the DC universe that possess god-like powers, through energy manipulation, magic ability, or technological advancement. Magic and the supernatural are often depicted as being real in the DC Universe, though some skeptics such as Mister Terrific maintain that there are scientific explanations to all such events. Magic is too powerful in the physical world, where harnessing magic can distort and even destroy reality if not properly controlled (i.e.: if the Lord of Order succumbs to certain events so will the Lord of Chaos).
The first beings calling themselves 'gods' first appeared billions of years ago on another planet, but they destroyed themselves in a terrible war. This unleashed the "Godwave," a wave of cosmic energy from the Source. This gave birth to other gods across the universe, including Earth’s. From the planet’s remains were formed the worlds of Apokolips and New Genesis, inhabited by beings that call themselves "New Gods". The Source Wall is an archetype of Buddha on the edge of the known galaxy. Certain speedsters believe in enlightenment in order to become part of the Speed Force (see below). Kismet is an immortal god and the embodiment of reality. She was matched with Marvel's Eternity in the JLA/Avengers. In mortal form she was member of the Lords of Order. Depending on the characters, other diverse religious deities from ancient cultures are common. Heroes such as Aztek and Black Condor, or villains like Black Adam, have found knowledge of their native roots in origin.
The Lords of Order and Chaos
These two groups of magical beings have been fighting against each other since the beginning of time, and they often empower others (with "Order Magic" or "Chaos Magic") in exchange for their acting as their agents. Many magical heroes and villains have been manipulated by them.
The Earth itself has a living spirit called "Maya" who, for millennia, has been creating champions, one for each of the mystical elements, to protect itself, using human beings as their hosts. Swamp Thing, Firestorm, Naiad and Red Tornado were some of them.
a subspecies of humanity with the natural ability to use magic, this race almost disappeared after too much crossbreeding with normal humans (it's from them that people in the DC universe inherited the ability to use magic.) The last pure blooded ones decided to retire to a magical invisible city centuries ago, and are now known as "The Hidden Ones". Zatanna knows many of the race's secrets, and Traci Thirteen is currently investigating magical and occult phenomena for the Croatoan Society—both women had Homo Magi mothers.
Like the Endless, these beings were created from the emotional energy generated by sentient beings. Each of the seven emotional manifestations of the DCU is represented by a different color (rage, red; greed, orange; fear, yellow; will, green; hope, blue; compassion, indigo; love, violet) and being. The various power-ring based corps of the DCU, most notably the Green Lantern Corps and Sinestro Corps utilize the energies of these beings.
Wizards and Sorcerers
Various sorcerers lurk in the DCU. Dr. Fate, Circe, the wizard Shazam, Mordru and Felix Faust are written as characters who use sorcery to create and destroy. Dimensions, rituals and spiritual realms are sources for magic power as seen in Ras Al Ghul's Lazarus Pit, Doctor Occult's use of the astral plane and the transformations of Captain Marvel.
Demonic entities vary from the Demon Etrigan, to Blaze, Satanus and Neron. Demonic entities are abundant and come from Hell although some like Eclipso, the vengeance demon (also referred to as the Prince of Darkness), reside on the Moon. Demonic Entities from Wonder Woman comics are directly linked to Greek Mythology such as Hades, and Ares. In the Vertigo comics, characters like John Constantine oppose demons influenced by Christian mythology. Most Demons are not however directly linked to Demonology.
Book of the Black
Technology more advanced than that which normally exists is available - but it is usually very expensive, and usually only rich or powerful individuals and organizations (or the scientific geniuses who create them) have access to them. S.T.A.R. Labs is an independent research outfit that often develops these devices, while Lexcorp is the main company selling them. It must also be noted that the government also runs the secret Project Cadmus (located in the mountains near Metropolis) to develop clones and genetic manipulation without the public's knowledge. Technology can also come from outer space or different time-lines. Apokolips weaponry is often sold in Metropolis to the criminal organization known as Intergang.
Robots and similar creations, including cyborgs, can have superior intelligence when they are created as sentient beings. The Manhunters, the Metal Men, Red Tornado, Robotman, Hourman, and Metallo are but a few examples. These 'beings' are most often created by individuals who possess vast intellects, like scientists Professor T.O. Morrow (maker of the Red Tornado), Dr. Will Magnus (who constructed the Metal Men) and Professor Ivo (who fabricated Amazo and other advanced androids using a form of Nano-technology developed by Lexcorp). Brainiac also emulates this technology as well as technology from other worlds. Similarly, some characters use technology to enhance their armor or modify cybernetic functions, for example Steel, Cyborg and the Cyborg Superman.
DC Database Prime Earth Article- http://dc.wikia.com/wiki/Prime_Earth
Wikipedia DC Universe Article- http://en.wikipedia.org/wiki/DC_Universe