The six realm theory of existence is a keystone principle in many Buddhist schools, especially the Vajrayana. Drawing from ancient cosmology, the six realms are a series of states which characterise and condition their inhabitants as Gods, Titans, Humans, Animals, Hungry Ghosts, or Hell-dwellers.
The practical worth of this system lies in its ability to parallel may of the mental states we enter throughout our lives, moment to moment. Each realm predisposes its inhabitants to particular behaviours: some make the practice of clear awareness and consciousness less hindered, while others flood beings with excesses of pain or pleasure distorting their natural flow of awareness.
Take it as a metaphor for self-evaluation; as most principles of its kind, it operates as a recipe to liberate the practitioner from conditioned states (As always, do not adhere too closely to the workings of this system or it will become habitual and hinder you more than it helps).
Here are some articles to start from:
The Japanese Buddhist Statuary
Buddha Dharma Education & Buddhanet