He's known through the entire universe as a great fighter and martial arts teacher. He lives on a small planet (in the Next Dimension at the end of Snake Way) that has ten times the gravity of Earth, making it a wonderful training place. If you train at King Kai's place for one day, its like training on earth for 100 days.
King Kai has a korny sense of humor and great wisdom. He has a great gift since he's able to communicate telepathically, which allows him to give his friends warnings in serious situations.
He teaches Goku the Kaio-Kan Attack, and the Spirit Bomb While he trains there.
King Kai is then killed in the Cell saga, and later becomes Goku's manager in the Outer World Tournament.