Namek is a small planet that is home to the Namekians, a peaceful race. The planet was said to be beautiful and covered in hydrangea forests. At some point in time, there was a great cataclysm and drought. It was believed that the Namekians did not survive, even by King Kai.
There were two survivors of the disaster, the Nameless Namekian (who would travel to Earth and split into Kami and King Piccolo) and Guru. Guru would begin repopulating the race on his own, getting their numbers up to a little over 100 by the time Frieza came looking for the Dragon Balls.
In the battle between Frieza and Goku, Namek was ultimately destroyed, but not before the Namekians were wished to safety. They repopulated a new planet, which they dubbed New Namek.