Son Gohan is first introduced in the first episode of Dragon Ball Z, he was about 5 or 4-1/5 at the time (In the dub he's 6 for some reason), and had a tail. His childhood wasn't an easy one, for the beginning part of his life, he was constantly studying because Chi-Chi wanted him to become a great scholar, Gokou, of course, went along with it. But when Raditz came and screwed everything up, Gohan's kiddie days were over.

His first trainer (Ok, while you poke-shit fans calm down and realize I'm not talking about that kind of trainer) was our favorite Namekseijin, Piccolo. Let's just say that Piccolo shouldn't become a full-time baby-sitter. Piccolo was rough on Gohan in the beginning because he knew of Gohan's potential, and he knew from the start that Gohan would become as powerful as the rest of the Z Senshi easily. And he did, in fact, Gohan became stronger than everybody when he became the first Saiyajin to go SSJ2.

Gohan's first transformation into Super Saiyajin took place in the Spirit and Time room with Gokou. In the anime, Gokou (while in SSJ) fired a Kamehameha at Gohan, Gohan knew that he had to become stronger in order to stay alive and to protect his friends. And he did, and he deflected the Kamehameha right back at Gokou. In the manga, it was a little different. Gokou continuly told Gohan that he had to become extremely angry, and after a couple tries, Gohan got very close, his hair was golden, as was his aura (well, it would be if it wasn't in black and white), but he couldn't maintain the form for long and dropped to the ground exhausted. Then Gokou was thinking about something (I can't read Japanese, sue me) and when he looked up he saw Gohan flying around the room in SSJ.

Gohan was the first Saiyajin to go beyond the bounds of Super Saiyajin, and into Super Saiyajin Two. SSJ Gohan got beat down after a close fight with Perfect Cell, then Cell stole the senzu beans from Kuririn and made seven Cell Jr.'s which proceeded to beat the crap outta the Z Senshi with no mercy (in the anime, the Z Senshi did a lot better, but in the manga they got completly destroyed and didn't even get one good hit on any of them.). Gohan whiched in horror as his friends were getting beated upon with no relief, then Cell had to go and kill 16-gou, this was the last straw and put Gohan over the edge, and into Super Saiyajin Two.

In the second half of DBZ (not really the second half, but after the Cell Saga and into the Tenka-Ichi Budokai Saga), Gohan is enrolled in Orange Star High School in Satan City, where he meets his future wife, Videl. Gohan also made an alias to fight crime as, Great Saiyaman. Complete with costume to hide his identity (his school mates won't find it easy to see Gohan blasting into SSJ, catching bullets, and throwing cars). His costume was made by Bulma, and was "capsule-ized" into his wristwatch. Gohan and Videl hit it off from the beginning, well sorta. Videl guessed that Gohan was Great Saiyaman. Later Gohan began to train Videl and taught her how to fly, I don't think he even bothered teaching her anything else. Soon after, things began to get interesting and they hooked up.

During the Majin Buu Saga, Gohan broke the seal on the Z Sword and released Rou Dai Kaio-shin, who in turn gave him a power up (after some encouraging from Gokou, hehe): Mystical Gohan. That "form" (which doesn't change anything physcially) was more powerful than Super Saiyajin (and I believe SSJ2, too.).

During DBGT, Gohan is your everyday business man, he even fights in full suit and tie from time-to-time. His mother got her wish, and Gohan was a sholar of some kind, I guess... Although he never went around training with his father like he used too. But Gohan was never really focused on at all in GT, another reason why it sucks; which is totally against what Akira Toriyama was trying to do in DBZ. Gohan is indeed powerful, but noooo, the writers of GT had to go ahead and make a new SSJ level and leave poor Gohan in the dust... bastards!!