（Purely personal perceptions, not necessarily correct ！）
The buddhists refer to the three realms as the desire realm, form realm, and formless realm. Taoism refers to the three realms as the heaven, human, and earthly realms. The six realms of existence are the Buddhist thought that the soul is immortal and that all sentient beings in the three realms will usher into different realms according to the good or evil they did while alive. Taoism also has the idea of the five roads of reincarnation and the six bridges, and the logic is similar to Buddhism.
The six realms of existence are: Heavenly Realm, Asura Realm, Human Realm, Animal Realm, Hungry Realm and Hell Realm. Among them, Heavenly Realm, Asura Realm and Human Realm are the three good paths; and Animal Realm, Hungry Realm and Hell Realm are three evil paths.
Although the Heavenly Realm is the best of the three good paths, it is still unable to avoid death and reincarnation.
Although the sentient beings in the Asura Realm have the powers of the gods, they do not have the virtuous behavior.
The Human Realm is the world we exist, and it is a great blessing to be a human being. If a person can practice the six perfections: generosity, commandment, patience, diligence, “keeping still and in deep meditation”, and wisdom, then that person maybe achieve Buddhahood through one’s efforts and insight and break from reincarnation.
Sentient beings in the Animal Realm suffer the least among the three evil paths, but they are too dumb and have difficulty practicing good karma, it is difficult for them to leave this realm.
Sentient beings in the Hungry Realm suffer a lot for not getting what they want.
Sentient beings in the Hell Realm are the most miserable. It is usually the ones who have done horrible karma that will be in this realm.
There are still many parts of the world that we haven’t learned yet, and we cannot judge whether a thing exists or not by our current knowledge, but this should not prevent us from exploring the spiritual world. Perhaps, life is just a consistent practice.