There are warrior Namekians and healer Namekians very rarely a Super Namekian is born, one with exceedingly high power.
Their appearance is green skin, pointy ears, and antenna.
They can: regenerate any part of their body as long as a small bit of it is left, a special few can create Dragon Balls and some (the healer namekians) can restore a being as long as they are not dead.
They give birth by spitting out eggs, which hatch eventually. Which is why Goku calls Piccolo a Yoshi. They are also referred as "Slug People".
The Namekians' home planet is called planet Namek.
They were wiped out once by a terrible storm however two survived, one a small child who was sent to earth to survive, the other a full grown Namekian who restored the entire race.
It is true that the Namekians don't have penises, as observed by Krillin.
They used to have a close relationship with their albino cousins, until Guru ordered a genocidal strike upon them. This is similar to the aggression between Green Martians and White Martians in DC Comics, which also are the publishers of Batman comics, which Super Kami Guru and Gohan are known to be fans of.