Gohan is the name shared by both the adoptive father of Goku and the son Goku named after him.
Adoptive parent of Goku and a student of Master Roshi. Gohan raised the boy alone in his home in the wild, giving him his first training in martial arts, until Goku became an Oozaru one full-moon-lit night and killed him. As he died, he made Goku promise never to look at the full moon again.
Son of Goku and Chi-Chi, named (after some argument) after Goku's adoptive parent. Gohan's half-Saiyan, half-human nature gave him a inherently higher power than either of his parents, though it only manifested when threatened while Gohan was very small. Chi-Chi intended to raise Gohan as a scholar instead of a fighter, but fate had other plans.
When Gohan was only four, just as Goku introduced him to his friends, Goku's brother Raditz came to Earth. Kidnapping the boy in the hopes of forcing his father onto a more Saiyan-like course, Raditz instead wound up causing Gohan to manifest his power, badly hurting Raditz and giving Goku and Piccolo a chance to defeat him. After Goku sacrificed himself to kill Raditz, the Saiyan warrior revealed that his fellow Saiyans would be coming to Earth; Piccolo took Gohan into the wild, hoping to train him in the use of his power for their arrival.
Gohan and Piccolo eventually developed a kind of friendship, one that eventually led Piccolo to sacrifice himself to save Gohan from an attack from Nappa. Gohan was later instrumental in the defeat of Vegeta, and accompanied Krillin and Bulma as they travelled to Namek, eventually helping them in their battles against the forces of Frieza.
Gohan later trained for the battle against the Androids, and later trained with his father in the Hyperbolic Time Chamber to deal with the menace of Cell. During that time, Gohan became a Super Saiyan, and revealed a potential beyond that level to his father; leading to Goku's eventual decision to have Gohan battle Cell when he could not defeat the villain himself. However, it was only when Cell attacked his friends that Gohan unleashed his rage, ascending to the level of Super Saiyan 2 and thrashing Cell. However, in his enraged state, he took too long, taking a cruel joy in harming Cell, which gave Cell enough time to initiate a self-destruct; and forcing Goku to sacrifice himself to save Earth. A guilty Gohan was soon forced to battle Cell anew, as he regenerated into an even more powerful form. In the Kamehameha battle that resulted, Gohan was able to overwhelm and destroy Cell with the encouragement of his father from the Other World.
In the seven years that followed, his brother Goten was born, and Gohan allowed his training to slip as he pursued his mother's ambitions for him, eventually coming to attend Orange Star High School in Satan City. While there, he eventually felt a need to use his power responsibly but secretly, adopting the identity of the Great Saiyaman. Despite the silliness with which he played superhero, the Great Saiyaman became widely acclaimed.
However, his classmate Videl eventually discovered his true identity, and blackmailed him into entering the World Martial Arts Tournament, as well as teaching her how to fly. Gohan's entry eventually led several of his fellow Z Warriors to enter, and also lured Goku to return from the dead for a day to enter as well. As he trained with his brother and Videl, he and Videl began to become close. During the Tournament, Gohan nearly lost control when the vengeful Spopovitch brutally beat Videl during his match against her. Unfortunately, his dispaly of power made him a prime target for Spopovitch and Yamu. They ambushed him during his battle against Kibito and drained his energy (assisted by the Supreme Kai, who paralyzed him), using it to bring Majin Buu halfway to reawakening.
Gohan was healed by Kibito, and after an abortive attempt to destroy Buu, the Supreme Kai took him to the Planet of the Kais and trained him in the use of the Z Sword. The Kai's ancestor, the Elder Kai, helped him ascend to the "Mystic" state, causing Gohan to reach his maximum potential; despite that, he was defeated by Super Buu, who had the powers of Piccolo and Gotenks at his disposal. Gohan was absorbed by Buu, increasing his power even more, but was later freed by Goku and Vegeta.
After the Majin Buu battle ended, Gohan spent some more time as the Great Saiyaman, but eventually settled into the role of a scholar, with Videl as his wife; they had a daughter, Pan.
Dragon Ball GT
Gohan was possessed by Baby when the alien invaded, eventually being used to wound Vegeta so that the Tuffle creation could merge with him. Gohan remained a pawn of Baby until the alien was destroyed by Goku. Gohan's role during the crises that followed was much lesser, but still significant.
Gohan was one of the most powerful beings in the universe, with combat skills learned from both Piccolo and Goku, and with such attacks as the Kamehameha and the Masenko Blast in his repertoire.
In Trunks' future timeline, Gohan was the only Z Warrior left, who regularly battled against Android 17 and 18. He eventually trained Trunks, until he lost an arm and was slain by the Androids in a climactic battle.
Note that while Garlic Junior is part of main Dragon Ball canon, the precise events of the movie Dead Zone are likely not, since Gohan's later manifestation came as a surprise.