When Kami was trying to become Guardian of Earth, he expelled all the evil from him. That evil turned into Piccolo Damioah or King Piccolo. Piccolo was born when kid Goku shot through King Piccolo's stomach. King Piccolo put his essence in an egg and shot it out of his mouth before he died. That egg eventually became Piccolo. In Dragonball, Piccolo was Goku's strongest and worst enemy. Later, when Piccolo and Goku teamed up against Raditz, Piccolo started to change. He has a special bond with Gohan and starts to fight on the good side. When Cell threatens to destroy Earth, Kami fuses back with Piccolo and they become a Super Namek.
Family
You could say Piccolo's dad was King Piccolo. Kami is kind of like Piccolo's brother, until they fuse. All the Namek's can be traced back to the same creator, probably Guru, so all the Nameks could be considered Piccolo's family.
Deaths
Piccolo died once in Dragonball Z. He jumped infront of a blast by Nappa that was intended for Gohan. Piccolo was wished back with Porunga during the Frieza Saga.
Goku
Goku is one of the 4 remaining Saiyans, an evil race of planet conquerers. His Saiyan name was Kakarott. As a baby, he was sent to the planet Earth to take it over when he grew up. He was found in the woods in a space pod by Grandpa Gohan.
Family
Goku's family name is Son, so that's why some people say Son Goku (in Japan, the last name comes first). Goku has a wife named Chi-Chi, and two sons named Gohan (after Goku's adopted father) and Goten. His father's name was Bardok. Goku also has a grand-daughter named Pan.
Deaths
Goku died twice in DBZ. First, he was killed when Piccolo used his Special Beam Cannon to kill Radditz. Next, when Cell was about to self-destruct, he teleported them both to King Kai's planet and was killed. Goku is brought back to life for one day by Uranai Baba (Master Roshi's sister) so he could be in the World Martial Arts Tournament.
