The Ferengi are a sapient humanoid species that originated on the rainy planet of Ferenginar. The Ferengi culture is wholly centered on economy, and their economy is centered in the principles of capitalism, with financial success being a Ferengi's primary goal in life.
Biology
Like most species in Star Trek, the Ferengi show a typical humanoid body plan, with main differences being either internal or located on the head. In the Ferengi's case, their most remarkable features are their extremely large ears, or lobes (bigger in males), which give them an audible range much greater than that of Humans and most other races. They have a large head, wrinkled nose, sharp teeth and their skin is orangy-brown. Their diet is omnivorous and includes invertebrates native to their homeworld (they will not eat insects from other planets, though) and an algae-based soft drink known as Slug-o-Cola. Ferengi were also the inventors of synthehol, a non-addictive substitute for alcohol.
The Ferengi's brain is essentially different from that of most other species. Their large skulls hide a four-lobed brain which cannot be properly read even by naturally telepathic species. Other traits include six toed-feet, blue nails, ascending ribs, two sets of lungs (upper and lower) and a particularly fast heart rate. They also have specialized cells in their blood called pyrocytes, which can cause severe allergic reactions on Human skin.
Like most humanoids, Ferengi have two sexes, male and female. However, their social norm dictates that females are restricted to being at home, and are not allowed to actively participate in business.
Culture and society
The principles of free market and the individual pursuit of profit are the pillars of almost every aspect of the Ferengi society, including their politics, religion, ethics and relationship with other species. Like the Vogons, they are also quite bureaucratic in nature, asking even casual visitants for formularies and admission fees. The Ferengi government is called the Ferengi Alliance and is headed by a Grand Nagus.
The Ferengi's ultimate business code is the Rules of Acquisition, which includes over 280 rules, number one being "Once you have their money, you never give it back". Although Ferengi ethics allow, and even encourage such things as distrust, greed and exploitation practices, they can not be truly described as an evil or immoral race. It would be more accurate to say that their moral code is very different from that of other species. Like Humans value ethics, Vulcans value logic and Klingons value honor, the Ferengi value profit more than anything else.
While early encounters with the Federation often saw Ferengi acting as pirates, thieves or smugglers; according to Quark, unlike Humans, Ferengi have never in their history engaged in such appalling practices like slavery, genocide or racial intolerance (one episode of Enterprise did have the Ferengi involved in slave market of other species, though). Also, of all races in the galaxy, Ferengi are among the least aggressive, which may have to do with their relative physical weakness, comparing to other species. They will always try to remain neutral in interstellar conflicts, a position that allows them to keep commercial routes with both sides. Ferengi esteem their purchasers and will try to maintain a good relationship with all other races.
Notes
For Star Trek: The Next Generation, the Ferengi were first intended to be a generic "enemy race" akin to the Klingons in the original series. In their first appearances they are depicted as much more barbaric, to the point that in their very first mention on screen, in the pilot "Encounter at Farpoint", Picard responds to the Groppler Zorn's comment about offering their services to the Ferengi instead of the Federation with the comment: "I hope they find you as tasty as they did their past associates". This implied cannibalism/barbarity was later dropped in favor of a more sophisticated, yet strangely profit-obcessed nature for the species.