Galactus is the sole survivor of the universe that existed prior to the current universe. Originally Galactus was a humanoid named Galan, who was born on the planet Taa, a paradise-like world whose civilization is said to have been the most advanced of any of the known universe of that time. Taa's civilization was one of the last still in existence. Lethal radiation caused by the "Big Crunch" this universe was experiencing was wiping out all life across the universe. Galan, a space explorer, was dispatched to travel through the cosmos to find a means of saving Taa, but he found none. The radiation eventually killed off all but a tiny fraction of the population of Taa. Knowing their deaths were inevitable, Galan proposed to the remaining survivors that they die gloriously by piloting one of their starships directly into the heart of the "Cosmic Egg." As the starship containing Galan and his fellow survivors approached the focal point of the Big Crunch, the heat and radiation killed all the passengers except Galan, who strangely found himself filled with new energy thus Galactus, the devourer of worlds, was conceived.
Galactus and Earth
Over the following millennia Galactus constructed for himself an immense home, the world-ship he calls Taa II, which engulfs the Archeopian solar system as a tribute to both his homeworld and to Archeopia, the first planet he ever consumed. For several billion years Galactus consumed only planets uninhabited by sentient lifeforms, Archeopia being the only exception, and centuries passed between his "feedings". However, as ages passed, the intervals between feedings decreased considerably in length, and Galactus found himself needing to consume worlds inhabited by sentient races if he could find no other worlds possessing the energy he needs in time to sustain himself. Galactus has succeeded in suppressing his sympathy for intelligent beings who are similar in nature to the entity from which he was birthed, and, realizing himself to be a higher kind of being than they are due to his intrinsic role in the natural order, is willing to destroy their lives to continue his own. Moreover, he is aware that he is meant to one day give back to the universe infinitely more than he has ever taken from it. At one point Galactus threatened to destroy the planet Zenn-La, home of a civilized, humanoid race. One of that world's inhabitants, Norrin Radd, persuaded Galactus to spare the planet by volunteering to become his "herald" and search out uninhabited worlds for him to consume. Galactus agreed, and transformed Norrin Radd into the Silver Surfer. Eventually, however, Galactus grew determined to consume the energy-rich planet Earth. The Surfer rebelled as a result, and Reed Richards of the Fantastic Four thwarted Galactus by threatening to use an alien device called the Ultimate Nullifier on him, a weapon that could lay waste to the universe and kill even Galactus. Galactus was forced to vow to spare Earth in exchange for the Nullifier's return, and he punished the Surfer for his betrayal by erecting an undetectable energy barrier that prevented the Surfer from leaving Earth. Eventually freeing himself from his pledge to Richards by defeating the nearly omnipotent Sphinx (who was further empowered by the secrets of the universe downloaded from the Living Computer of Xandar), Galactus returned several times to Earth to consume it, but was always narrowly thwarted. Meanwhile, he consumed numerous inhabited worlds throughout the known universe to sate his hunger, and was feared as a menace to all known star faring races. Galactus also took on a succession of new heralds. During his last attempt to devour Earth, Galactus had gone so long without "feeding" that his energies were the lowest they had ever been and he was near death; thus a large number of Earth's superhuman defenders actually succeeded in defeating him in battle. Galactus lay dying, succumbing to his own hunger, but Reed Richards saved his life with the help of Thor's mystic hammer Mjolnir. The grateful Galactus pledged his friendship to Richards and sincerely gave his word never to attack Earth again. Galactus has steadfastly abided by this oath ever since. During this visit to Earth Galactus recruited his latest herald, Nova.
Galactus' Superpowers
Levitation: Galactus can make himself float and move through the air.
Size-Alteration: Ability to alter the physical size of persons or objects.
Molecular Restructuring: Ability to physically alter persons or objects through the manipulation of their comprising molecules.
Matter Transmutation: Ability to directly convert one form of matter into another form of matter. e.g. turning lead into gold.
Cosmic Teleportation: Ability to teleport beings and objects—even entire galaxies—across space and time. This ability also allows him to teleport himself.
Cosmic Force-Fields: Ability to erect completely impenetrable energy fields.
Cosmic Energy Projection: Ability to project energy with incalculable force.
Interdimensional & Intradimensional Portals: Ability to create wormholes between dimensions and within dimensions.
Cosmic Telepathy: Ability to read or communicate with other minds across universal distances.
Cosmic Telekinesis: Ability to move virtually unlimited amounts of mass through mental concentration.
Cosmic Awareness: Ability to sense disturbances or changes within the cosmic ether on a universal scale.
Resurrection: Ability to raise the dead.
Creation: Ability to create complex, sentient, biological life-forms from nothing.
Soul Manipulation: Ability to manipulate the souls of living beings.
Recreation: Ability to remake even an entire Solar System, including populations, the planets etc in every detail.
Power Bestowal: Galactus' heralds, whom he has endowed with a minuscule fraction of his cosmic power, are able to manipulate matter and energy in ways far beyond human comprehension.
Vitakinesis: Galactus is able to heal himself and others from all physical wounds although this may simply be molecular reconstruction.
information from marvel.wikia.com