"Dhyana" is Sanskrit for absorbing meditation, that is a form of meditation where the mind becomes turned inwards and stills itself. In Pali, the term is "Jhana". Ch’an and Zen are derived from this word, and make the practice of meditation their central focus. This is split into levels of absorption, according to the properties depth of the state acquired. The traditional Buddhist way of categorising these states are:
The Realm of Desire
The normal state of consciousness of beings that live in the realms of desire, including humans, animals, hell beings and Pretas. Here, the mind is buffeted by desires and cravings. Concentration and mindfulness are not present in a strong degree since the mind is constantly jumping around and is submitted to various comings and goings of the phenomenal world.
Dhyanas of Pure Form
These are the states that a meditator first reaches. There are still elements of thoughts and desires, but these are not strong, and get weaker as the 4th Dhyana is approached. Discursive thought is still possible in the 1st Dhyana, but it is clear and un-muddled. They are also the Heaven realms of the Gods of Pure Form.
1st Dhyana - "Joyful Stage of Leaving Production"
2nd Dhyana - "Joyful Stage of the Arising of Samadhi"
3rd Dhyana - "Stage of the Wonderful Bliss of Being Apart from Joy"
4th Dhyana - "Stage of Renouncing Thought"
Dhyana of Formless Realms
As well as being meditative states that humans can reach, these describe the formless Heavens that formless Gods inhabit. Although these are very subtle states of being, and are beyond any really meaningful description apart from the sketchy ones given, they are not considered full enlightenment, and must be gone beyond for the full realisation of enlightenment. Sometimes a 9th stage is added to this list that indicates a state that once reached, is the realisation of enlightenment.
5th Dhyana - "The Sphere of infinite space"
6th Dhyana - "The Sphere of boundless consciousness"
7th Dhyana - "The Sphere of nothingness"
8th Dhyana - "The Sphere of no thought, no no-thought"
The first Dhyana (and sometimes the second) are often experienced by people when they are very calm and happy, and when a person is totally absorbed in something such as painting, writing or playing sport etc., they will slip into this state. The later ones need an actual practice of meditation to achieve them - they are too subtle to stumble acros naturally.
Although they are a guide to the state of meditation, they should not be thought of as separate levels, but rather as one long spectrum of experience.