History and Politics
Betazed has enjoyed a relatively untroubled history for the last few centuries. This peaceful existence came to a halt in 2374, when the Dominion invaded and occupied the planet. It is probable that the Dominion occupation ceased after the end of the Dominion War. (DS9: "In the Pale Moonlight")

According to the novel "The Battle of Betazed" the Betazoids were finally able to drive the Jem'Hadar away by the application of a highly dangerous, offensive form of empathy.

Physiology
Externally, Betazoids are among the most humanoid species on record and are outwardly indistinguishable from Humans in every aspect but one, the irises of their eyes are completely black. Betazoids are natural telepaths; most develop this ability in adolescence, but a few are born with their telepathic abilities already active. Unable to screen out the noise of other peoples minds, these individuals generally suffer mental problems of varying severity depending mostly on when the problem was diagnosed.

Betazoids can cross-breed with Humans, although this often affects the telepathic abilities of the offspring - most commonly the children of such a union develop empathic abilities as their primary psionic talent, while their telepathic abilities, though existing, are rather below average for Betazoids. Usually the telepathy of these half-breeds, without extensive training, is limited to communication with other empaths or telepaths and full telepathic contact with emotionally very close persons (Imzadi). All full Betazoids are unable to read the thoughts of Ferengi, Breen, Ullians or Dopterians, but some half Betazoids have been occasionally successful in sensing the emotions of some of these species. Additionally even full Betazoids seem to be able to psionically influence some of these species (TNG: "The Price", "Ménage à Troi"; DS9: "The Forsaken", Fascination")

Most Betazoids have telepathic capabilities, and most are from weak to medium power. There are, however, a few especially strong telepaths. The capabilities develop in adolescence, and most receive formal training for their power based on strength. They are all different, and sometimes a Betazoid can be born with full capability. This poses strong problems, and often the individuals are hospitalized and even isolated. (TNG: "Tin Man")

Betazoids reach rapid eye movement (REM) sleep at a different frequency from other humanoids. (TNG: "Night Terrors")

In mid-life, Betazoid females undergo a physiological change known as "the Phase" where their sex drive quadruples or more. (TNG: "Manhunt")

Mature Betazoids can also suffer from Zanthi fever, that causes the Betazoid to loose control over the projective aspect of his/her empathy. That can result in people around that Betazoid acting as if they'd experience the same general emotional state as him/her, though they are directed by their own subconscious desires, fears etc. (DS9: "Fascination")

Culture and Tradition
Betazed has a complex hereditary nobility; prominent Betazoid diplomat Lwaxana Troi, for example, is "Daughter of the Fifth House, Holder of the Sacred Chalice of Rixx, and Heir to the Holy Rings of Betazed." Betazoid tradition has children genetically bonded to a future spouse. (TNG: "Haven")

In the Betazoid wedding ceremony, all participants are traditionally naked, not only the soon to be wed, but the guests also. (TNG: "Haven", "Cost of Living", Star Trek: Nemesis)

At one time, it was fashionable for Betazoid women to wear elaborate wigs that encaged small animals. The practice, which was cruel to the animals, was stopped when one (unknown) woman stood up against it. (TNG: "Half a Life")

It has been speculated that Betazoid society in the past was matriarchal. (TNG: "Angel One")