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.