Joey is Yamcha's son. He is a full human. Unlike Yamcha(who palys basseball now), Joey is a fighter and he is a football fan. Joey was at a football game one day and saw players tackling each other. Yamcha was with him and mentioned that he used to fight people from all over the galaxy. After that Joey joined the kids football team as a sort of "training" for him. As Joey trained he realised that football was too easy for him because he was a fighter and he was stronger than everyone else. Then Joey took on wrestling. He was still stronger than the other kids so he played with the teenagers and soon the pros. Eventually even they were too weak for him. So Joey trained with his dad for a while. He moved on to Master Roshi, Krillin, Tien, and Chiaotzu. Finally before moving on to the Z-Fighters Joey trained with Mr.Popo and Dende. After Joey decided that he was one of the strongest humans he went off to the Z-Fighters. Goku, Vegeta, Goten, Toad, and Trunks each had a turn training him so he had a variety of skills. Joey stayed with the Z- Fighters and eventually he left the group. Joey died at the age of 19. He died so early because when a new threat came to the Earth it wanted to kill the warriors from weakest to strongest and Joey was weaker than the other Z-Fighters.
Joey's power level as a kid is the same as Goku right before training with Kami. After football and wrestling his power was Goku's after training with Kami but before King Piccolo. After completing his training his power level is as strong as a SSJ2.