What are the Three Realms of Existence in Buddhism?

Three Realms of Existence

Three Realms of Existence refers to the desire realm, form realm and formless realm. It is also called three kinds of existence. As the confusion and suffering in the three realms are as boundless as the ocean, it is also called Bitterness Realm or Sea of Bitterness.
(1) Desire realm (Sanskrit: kāma -dhātu), namely, the world with lust, sexual passion, eroticism, desire for eating，etc. It got the name because from the Paranirmitavas/avatin-deva (the King of desire realm and the of prince of devils)in the immortal realm to the four continents in the realm of human existence and the twenty places in the relentless hells, men and women live together with multiple desires. Explanation: desire here refers to five desires: the desire for wealth, lust, fame, food and sleep, with the focus on the sexual passion between men and women. Just as the saying goes: “Men and women live together with multiple desires; thus, the realm is called desire realm.” There are five trends and twelve places in the desire realm. Trend means where to go; and the five trends, also called mixed living places, indicate that one has five places to go to after death according to the goodness and badness they’ve done in his lifetime, namely, going to the hell, being an animal, being a hungry ghost, being a human being or the Six Heavens of Desire. Sometimes, Buddha and Bodhisattva come here to save people from suffering, and some of them even become immortals. Thus, the place is also called the mixed living place between human beings and immortals. The hell is composed of eight places: Samjiva, Kalsotra, Saṃghāta, Raurava,Mahāraurava,Tapana and Pratāpana. Together with Brute and Hungry Ghost, they are called ten demoniac trends. The realm of human existence has four places, which are Jambudvipa, Purvavideha, Aparagodaniya and Uttarakuru. The heaven has six places, which are Heaven of the Four Great Kings, Heaven of the Thirty-Three, Free of CombatJoyous Realm, Enjoying Emanation and Controlling Others' Emanations. The realms of human existence and heaven together have ten virtuous trends. Humans live here. The hell is beneath the Pratāpana at the bottom. The heaven is above, with the heaven of Controlling Others' Emanations at the top.

(2) The form in Form realm (Sanskrit: rūpa -dhātu) means to become obstacle or to appear. It is a realm far away from the desire realm’s lust and desire for eating but still has quietness and form for sentient beings. This realm is above the desire realm and not infected with desire. Without any female beings here are produced by metaplasia; its palace, produced by the metaplasia of form, is magnificent. Everything here is especially excellent and exquisite.It is called form realm because it still has from. This realm is divided into four levels according to the depth and subtlety of the state. There are 18 heavens starting from the beginning level of Brahma and ending with the ending level of Akanis!t!ha-deva. Explanation: form realm is above the desire realm. It is the residence for living creatures without desires. Form means material. “The magnificent palace is produced by the metaplasia of form, so it is named form realm.” Form realm has four calm contemplations and 17 levels of heaven. Calm contemplations mean the meditation state of calmness and contemplation. Four calm contemplation levels of heaven are the results of different meditation depth of sentient beings. It has four grades of residence. The beginning calm contemplation has three levels (Brahma-parisadya-deva, Brahma-purohitaand Brahma). The second calm contemplation has three levels (Parittabh, Apramanabhaand Abhasvara). The third calm contemplation also has three levels (Parittasubha,Apramana-subha andSubhakrtsna). The fourth calm contemplation has eight levels (Anabhraka, Punya-prasava, Vr!hatphala, Avrh, aAtapa, Sudrsa, Sudarśana and Akanistha). These eight levels of heaven have both ordinary and sage inhabitants. The first three levels are four ordinary inhabitants while the rest five levels are for sage inhabitants.
(3) Formless realm (Sanskrit: arūpa -dhātu) only has four thoughts of bearing, thinking, doing and knowing, a sentient world without materials. This realm does not have any material. It does not have any body, palace or territory, either. Only mind and thought live in deep and mysterious meditation state, so it is called formless realm. This realm is above form realm. It has four levels of heaven (Akasanantyayatana-deva, Vijñānānantyāyatana, Akicanya-y and Naiva-sajjñānāsajjñāyatana). They are also called four formless or four empty levels. Explanation: Above the form realm is residence for formless inhabitants. “It has only four thoughts but does not have any form, so it is named formless realm.” Formless realm has Akasanantyayatana-deva, Vijñānānantyāyatana, Akicanya-y and Naiva-sajjñānāsajjñāyatana, which are all named after different mind states. Formless realm does not have a place. Affections stay at some place and space, leave the body behind and form the realm at the space. It is also named four empty levels of heaven because it does not have any form. These three realms are generally regarded as one world. Mahayana believes there are countless such worlds in the universe.
These three realms, having different results in terms of their quality, happiness and suffer, are defined as confusion realms where all living creatures experience eternal cycle of birth and death. Therefore, sages dislike and abandon them. Saddharmapundarika Sutra·Opammavagga (big nine four latter), “The three realms do not have any peace. They are like a burning house; it is filled with all kinds of bitterness, and is really frightening.” Imaginary City (big nine·two four latter), “Inhabitants of the three realms please work hard to emancipate all living creatures from the three realms.”This is to admonish sentient beings of the three realms not to indulge in the realms but to seek liberation instead. [Soutern Version Mahaparinirvana Sutra Scroll Thirteen, Abhidharmakosa-sastra Scroll Eight, Prakaranapada Scroll Five, Mahavibhasa Scroll Seventy-Five, Yogacarabhumi-sastra Scroll Four, Mahaprajnaparamita Upadesha Scroll Twenty-One and Avatamsaka Sutra Contents Scroll Two]