Piccolo was the evil half of Kami. Piccolo is a Namekian. Originally King Piccolo or Piccolo Damioah, he terrorized the planet until Goku did a flying headbutt through his stomach. Piccolo Damioah then preserved his essence into a egg, spit it out then died. The egg hatched the Piccolo we all know today. Piccolo was Goku's arch enemy until Raditz came and threatened the Earth. Piccolo was the one that killed Raditz and Goku. Piccolo also saw Gohan's hidden power and trained Gohan for a year to help fight the Saiya-jins, Vegeta and Nappa.
Piccolo was killed by Nappa, but Piccolo recieved training in the afterlife along with Yamcha, Tenshinhan and Chaotsu from King Kai. Piccolo was wished back to life by the Namekian Dragon Balls and was transported to Namek to help fight Freezer. Along the way Piccolo fused with another Namekian warrior Nail, and gained huge power. Piccolo took down Freezer in his second form, but was outmatched afterwards.
During the Cell saga Piccolo once again merged with his Namekian counterpart Kami. Piccolo was now as strong as Android 17. However, Piccolo couldn't stand up to Cell.
Piccolo now plays a smaller role in DBZ than he did originally, but still remains a good fighter.