Galactus was a weapon used by the Skrulls, an anti-life virus. It was created by them using Mephisto's world. It traveled the cosmos eating worlds. By the time it attempted to destroy Earth, it had already destroyed at least a thousand worlds. The planetoid was able to take incredible levels of damage. Mr. Fantastic's defense barrier consisting of a network of satellites that surrounded Earth and equipped with gamma missiles and mega plasma cannons (powerful enough to shatter a planetoid's core) had no effect on Galactus.
Setting its sights on Earth, the Fantastic Four took measures to stop Galactus from devouring their world by recruiting Marvin Ellwood, the heir to Captain Marvel to go onto the surface of the world eating monster, where they themselves could not survive, and kill it. At some point in the past it swallowed the Inhumans' world of Attilan. The survivors ended up on Galactus and since then were waging a war with the inhabitants of Galactus who were led by a Skrull (Lord Kreega) which included Lava Men (equipped with insectoid-like walkers and magma cannons) and the planetoid's own symbiotic lifeforms. When the Fantastic Four and the new Captain Marvel assaulted Galactus, they were aided in their battle against the defenders by the Inhumans. Only to discover the heart of Galactus being guarded by the Skrull's gods, each over a hundred ft tall. In a final confrontation with Lord Kreega, Captain Marvel defeated him and sent him into Galactus' heart, killing the planetoid.
Powers and Abilities
It was surrounded by a field of asteroids and was capable of bombarding spacecraft (e.g. orbiting defense space stations) and a planet's surface with meteorites. Using its giant tentacles which borrowed into a planet, it released symbiotic parasites ranging in size from microscopic level to 100s of ft tall. Ravaging the world's natural resources and draining it of life.
An energy core deep within in its inside, which acted as its living heart.