The Namekians are an alien race of green slug-like humanoids from the Planet Namek.
In the era of the Dragon Ball series, there were just over 100 Nameks alive on their home planet; they had been killed off a generation before by a great tempest. The only known survivors are Guru, Lord Slug and Kami, who was sent to safety on Earth.
During the Namek Saga, their homeworld Planet Namek, was terrorized by Frieza and his henchmen, Dodoria and Zarbon. The planet was ultimately destroyed by Freeza as a last-ditch effort to kill Goku in their battle during the Frieza Saga; Frieza could survive in space whereas Goku could not. All of the Nameks were temporarily transported to Earth, where they lived for nearly a year before being resettled on New Namek.
What is known of pre-series Namek history is very limited. Three-hundred years prior to the beginning of the series, the earliest Nameks encountered on Earth (King Piccolo and his sons, though not counting Kami) were evil and destructive and caused chaos throughout the Earth until their eventual narrow defeat at the hands of Mutaito and his pupils (including Master Roshi and Master Shen).
It is also clear that they were a well-known spacefaring race at some point in the not-too-distant past as the Saiyans who came to Earth immediately recognized Piccolo Jr. as a Namek. It is extremely unlikely that they would be so well-known, of the many alien races introduced over the course of the series, if they were only a primitive farming culture on a single planet. It is also known that Nameks had higher technology because Kami was spirited away to Earth in a space ship (with Namek-language voice recognition) prior to the disaster. Also, Lord Slug from movie #4, who is apparently older than Kami, was an avid space-traveler, and was most likely exiled from Planet Namek prior to the massive climate shift.
Up until the arrival of Raditz, Piccolo Jr. and the other Nameks on Earth were considered Mazokus (demons) due to the evil nature of King Piccolo and his sons. Kami's status as a Namek was not generally known, though he would not have been considered a Mazoku. Some point after Kami came to Earth he lost memory of his homeworld, and thus, Kami forgot he was even an alien. It is never explained if King Piccolo knew he and Kami were Nameks, but he was shown using what was later revealed to be Namek language when talking directly to his sons. If King Piccolo knowing the language is because of him knowing his origins, there is the possibility of Kami's evil originating from being a Namek warrior, or from his father Katas.
King Piccolo had a number of animal-like sons who were considered Nameks due to retcon. It is possible that King Piccolo's evil caused his own sons (including Cymbal, Drum, Piano, and Tambourine) to be mutated and malformed, so much so that they didn't even resemble normal Nameks.
Nameks are humanoids with slug-like characteristics, including antennae, light green skin with pink patches for young adult and kid Nameks, while the elderly Nameks have dark green skin and pink patches (these get dull-orange as the Namek advances in age), pointy ears, and red rings, in which they have three red conjoined rings on their ankles and wrists, making them look like they have ankle bands and wristbands. They require only water for sustenance, though Piccolo Jr. once ate a fish, and are adapted to a world with three suns, with at least one up at all times.
They do not have genders and reproduce asexually by spitting out an egg (though this process usually shortens their lifespan) but are generally identified as males. The sons are not usually genetic copies of the father like with Piccolo Jr.; rather he appears as if the father has some control over what type of child will be formed. This is carried to the extent that they may be mutated and not closely resemble others of the species. King Piccolo was able to spawn demonic animal-like Nameks that did not resemble a Namek at all to help him in his quest for eternal youth and complete domination. It appears Nameks are usually close to their children, and though King Piccolo treated his sons as minions he appeared to care for the sons he gave life in his old age. (In contrast, he was seemingly unfazed by the deaths of his sons at the hands of Mutaito and his students, as well as Drum after his youth was restored.) Nameks are also telepathically linked to their sons and can reach them from seemingly anywhere, as well as sense their deaths.
Their blood color is purple, though earlier on King Piccolo and Piccolo Jr.'s blood was portrayed as red. Drum is the first Namek ever to be portrayed with purple blood, however.
Most Nameks appear to be roughly the size of normal humans, but King Piccolo, Kami (mostly his height), and Guru are all shown to have a height and size beyond normal humans, this is never explained.Possibly ancient Nameks like Lord slug were shown much taller than regular Nameks.
Though generally not as strong as Saiyans, they are quite capable combatants.
In the series, Nameks demonstrate a wide variety of abilities, many of them magical in nature. These abilities are seen mostly by important characters, such as Piccolo Jr., so it is unknown how many are common to the species versus the individual. It is likely that the vast majority of them are restricted to fighter-type Nameks as opposed to sage-like Nameks , as none of the normal Nameks displayed the ability to use them when facing the crisis of Frieza's invasion.
After Piccolo Jr. fuses with Kami, he powers up to levels well beyond of the existing Nameks. Thus, he is called by Krillin a "Super Namek" because of his new and extraordinary powers, able to far eclipse the strength of an untrained Super Saiyan.
In Lord Slug, some extremely powerful Nameks found the ancient secret to unlocking a higher consciousness within, becoming "Super Nameks". These Super Nameks were extremely aggressive and very different from their agrarian counterparts. These Super Nameks also seem to be able to show off their true power. They were banished from the planet using the Dragon Balls. The only example shown of such is Lord Slug.
It is doubtful that Super Nameks are a fundamentally different type of Namek. Super Nameks, as far as can be discerned, are simply very powerful Nameks, and the term "Super Namek" is only a nickname.
By the end of the manga, Piccolo Jr. is by far the most powerful Namek, which would make him much more powerful than the "Super Namek" Lord Slug, or any of the others that are known of in the series. Piccolo Jr. could hold his own and even take the advantage at points in battle against an opponent as powerful as Android #17, during the Android Saga. As the Cell Games approach, Piccolo Jr. decides to enter the Hyperbolic Time Chamber for training. Doing so also greatly increases his power level, as noted by Goku and Trunks. The argument is that Piccolo Jr. fused with two powerful Nameks (Kami and Nail), making him by default the most powerful Namek. Even without fusing with Nail and only fusing with Kami, Piccolo Jr. would still be much more powerful, as noted when Nail states that if he had been reunited with Kami prior to coming to Namek, he would be more than ready to defeat the same Frieza that mauled Nail in battle.
Only plot-important Nameks are shown or described as being one of the above castes. It is unknown if the other Nameks in existence belong to one of them or to other unmentioned castes.
Piccolo Jr. could be considered a half Fighter-Type and half Dragon Clan Namek hybrid because of his vast abilities. With a mix of Fighter-Type and Dragon Clan. He is an excellent combatant and has many Dragon Clan type mystical abilities. He also is the most powerful Namek by the end of the series, which should place him in an entirely different caste of Namek.