The Ruin (破滅, Hametsu) is a cataclysmic event that quite literally killed the world and the majority of its life. After Casshern saved Luna, the life of the world, the Ruin stop and toxic gasses were released into the atmosphere killing many Humans and causing Robots to rust prematurely. Most plant life and sources of water were also dried up leaving the world a barren wasteland.
The World was a place ruled by Braiking Boss' robot empire who hunted humans in order to establish Robotic absolution. But Humanity began to look to a new leader, Luna who could promise salvation from Braiking Boss' tyranny. Braiking Boss believed that the world could not have two great leaders and ordered his most powerful Robot, Casshern to save Luna. Casshern did so, he recuse Luna and mixed their blood, causing life and death to switch roles and threw the world into Ruin.
The Ruin is an apocalyptic event that threw the world into a state of eternal decay. Beings who were once thought immortal were now at the mercy of incoming death. Robots were "dropping like flies", filled with iron-oxide from the poison atmosphere, causing them to rust.Even humans, who gained eternal life through advancements in medication were sickened by the poison which caused them to perish as well.Robots and Humans alike were now on the same level in a sense because they now knew death without the escape of eternal life. This was all caused by the death of Luna, when Casshern recuse her, their blood mixed and turned the world on itself, switching the roles of life and death.
- Robots believed they could escape the Ruin by devouring Casshern. This was only partially true, it is revealed by Dio that if someone could kill Braiking Boss, they could find out whats inside him that allows him to be immortal. 
- It was never stated why some Robots such as Jin and Friender were unaffected by the Ruin, but Dai and Tetsu jokingly stated the point of their firm convictions, which could be the reason they are unaffected by the Ruin. Their unbending beliefs were quite possibly unaffected by the turning of the world causing them to be unaffected by the Ruin itself. 
- "Ruin" is also a term in the anime that is used as a synonym for "Death".