The prawns, aliens, outlanders, or non-humans of District 9 are a spacefaring race of sentient beings, with highly advanced technology and spaceship engineering. Their reasons for coming and/or colonizing on Earth are unknown so far. The premise of the film is essentially humans vs. prawns, which is a blatant reference to the Apartheid Era of South Africa in the 1990's, but instead of racial clashes, it represents clashes between two species.
According to David Meng of Weta Workshop, the vast majority of the aliens are worker-class "drones", explaining why they were so lost and ineffectual. Christopher Johnson is one notable exception to this rule.
(None of this non-human social structure made it into the film; though, one biologist interviewed in-universe did offer speculation that they were exactly that. Perhaps director Neill Blomkamp is saving it for a future film?)
Speculation on Non humans in the film
There's a possibility that because of the uniquely hominid shape of their bodies and comprehension of English that their society is derived from humans that have been captured and metamorphosed into prawns by the same liquid which is the basis of the main character's transformation. The prawns then would actually be a representation of humans taken out of society and allowed to experimentally create a society and language out of scratch from perhaps 1950's era abduction stories and UFO interest having been people that have actually disappeared to become the experimental culture-less prawn population, which they clearly do not do (Which could also be seen as a reference to the American goverment's actions on indigenous Americans in that they were placed into concentration camps and forced to speak in English and live as they were told, which would be the same as simply losing an entire society's thousand or more years of social and cultural evolution). This and the fact that the transformation that the liquid creates onto the main character is a very complex biological one which would be literally impossible as science currently understands life and so might hint that the liquid is created for the sole purpose of it, with a double purpose of being a fuel.
An alternate possibility is that the fluid is a form of repair nanotechnology, something that fixes things and wilkus' transformation is a result of the fluid "correcting his human genetics into that of the prawn-like nonhumans.
Possible connection to Alive in Joburg
Also, there is something else unconfirmed by director Neill Blomkamp: In his previous film "Alive in Joburg " which inspired District 9, the aliens referred to themselves as Poleepkwa. Interested parties have contacted the director on whether or not this term can be officially applied.
The non-humans have a variety of extremely-powerful energy weapons. The weapons can only be activated by alien DNA, which means that no human can use them (with the sole exception of Wikus). The weapons shown in the film are mostly energy-based, but several clips prove otherwise, for example, at one point, Christopher can be seen using one that appears to handle like an assault rifle when he and Wikus stormed MNU headquarters. Wikus uses a gun in the Nigerian gang base capable of producing a shockwave blast powerful enough to blast a human through a wall at high speeds. Even a near miss can send someone flying through the air as shown in the film. Another weapon of note is a large alien mech that Wikus piloted near the end of the film. The suit appears to have a number of other alien weapon capabilities integrated into it, along with a magnetic field generator that allows it to catch enemy projectiles and launch them right back at the enemy.
The most well known weapon is the ARC gun. It fires a huge charge of electricity and rapid molecular expansion of gas and fluid. In a soft biological target, an ARC gun strike can result in a catastrophic explosion of blood and gore. But even a near miss can cause superficial burns and keep an enemy subdued.
The Homeworld of the Prawns is mentioned to have 7 moons, it is also noted by Christopher to his son that it is several times bigger than Earth. Due to the fact that the Prawns can breathe Earths oxygen atmosphere, it is very likely that their Homeworld has very similar conditions as on Earth. This would also explain why the Prawns landed on Earth instead of any other nearby planets on their route.
However no hints are made in the movie where this Homeworld is exactly located or whether it is the only world inhabited by his species. At the end of the movie, Christopher can be seen guiding a navigational system in his craft, which shows a map of several galaxy-like structures, implying the Prawn Homeworld may be not located in the Milky Way. It is also shown that the Homeworld is located in a globular cluster.
Prawns, unlike humans, eat almost all of their food raw. They appear to be primarily carnivorous, as per the large amount of meat dealers in District 9. They also have a very strong liking of cat food, which has an effect on the aliens that is similar to the effect of catnip on cats. In many deals with the local human gangs, Prawns will trade their weapons and technology in exchange for cat food. When purchasing their food, the prawns are more often than not ripped off. Dealers overprice the meats and cat food, making it hard for the non-humans to get food. When buying food is not an option, they will rummage through garbage in search of anything edible. Another unusual eating habit is their enjoyment of rubber. Prawns have been seen fighting over car tires because they enjoy the taste and texture of this material (Wikus explains that chewing car tire rubber is sort of like "a big marshmallow" to them, but whether they can actually digest the rubber, or gain nutrition from it, is unclear).
Anatomy and Physiology
The non-humans are bipedal, several feet taller than humans, and have dark thick shell-like skin as well as a mass of facial tendrils. It is hypothesized that these tendrils are somehow involved in the mastication process, but it has not been proven. These aliens are also very strong. A non-human has been observed taking hold of an MNU soldier's arm, and placing his foot on his chest. He kicks outward, resulting in the soldier's arm being ripped off, as well as sending him flying several yards back. In another case a non-human was observed breaking a shotgun in half with a single one-handed blow. Physically speaking, they are toothless, and use a variety of feeding tendrils along the edge of their face to help quickly feed them. They are opportunistic carnivores and will eat just about anything they can come across, although for currently-unknown reasons wet cat food affects them in a manner similar to how catnip affects a cat. Unfortunately for them, this addiction has caused them to lose much in trades with the local Humans. This cat food addiction is used against them as a racist stereotype.
Their bodies are covered in a tough exoskeleton, and they are also notable for their three-fingered hands, large humanoid eyes, two pairs of antennae, and apparent inability to speak Human languages, although they can comprehend them. When young are born they appear to soon be capable of fully understanding their species' technology and also appear to undergo an incomplete metamorphosis, most likely shedding their exoskeletal carapace over time in order to grow in size.
Prawns reproduce asexually, meaning each individual possesses both male and female reproductive organs, and are capable of self fertilization. This makes their reproductive capabilities somewhat threatening, and has allowed their population to increase quickly, despite MNU attempts to limit the amount of offspring produced by aborting eggs and licensing young.
Once an egg has been laid, it will be connected to a food source (such as a dead cow or pig) via tubes, and in this way the developing Prawn will be nourished. Parents can be extremely protective of their eggs, and MNU officers avoid aborting while the parent is present to prevent violent confrontation. It is worth noting that the presence of inter-species prostitution within District 9 indicates that Prawns are capable of intercourse (as are most hermaphroditic species of Earth, ie. snails), and may have some reproductive structures similar to humans.