Orions are known for their distinctive green skin. Orion males are typically bald and, on average, taller and more muscular than an average Human male. Orion females are very animalistic in nature, known for their extreme carnal appetites and their innate skill of seduction: they are kept in slavery by the males. The Human male can rarely resist the alluring dance of an Orion slave-girl.
At least that was the position of the traditional Star Trek canon until Enterprise. In the Star Trek: Enterprise episode ‘Bound’ their slavery was revealed to be only a pretence that allows them to conquer other species by deception. The females are sold on the Orion slave market allowing them easy access to new targets for their unique physiology: very soon the ‘owner’ begins taking orders from his new ‘slave’. As Enterprise was a prequel, this revision of standard history sat uneasily with the way later Federation crews had interacted with Orions.
This revision was itself (apparently – unless her ‘true’ nature remains to be revealed) subverted in the rebooted Star Trek movie where Gailawas said to have escaped the life of an Orion slave girl through ‘an underground railroad’.