From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Cell Games (セル ゲーム, Seru Gēmu) is a tournament that Cell creates in order to show his power to the world. Cell gives an international announcement for the games on May 17, 767, and gives the fighters who want to participate 9 days to prepare. The Earth's Military tries to stop Cell before the tournament begins, but fails. Mister Satan announces that he will defeat Cell at the Cell Games.
Nine days later, on May 26, the Z Warriors have done as much training as they possibly could, except Goku and Gohan, who was convinced that Goku was enough of a match in his Full-Power Super Saiyan form. When the tournament commenced, Goku decided he would be the first one to fight Cell.
However, the first challenger to arrive at the Cell Games was the relatively weak human Mr. Satan (winner of several Tenkaichi Budokais the Z Fighters did not enter) and challenged the android himself along with his students. Cell proves to be far superior to them as expected, and they are quickly disposed of. Vegeta is the first to arrive of the Z Fighters, and is soon followed by the rest.
Goku was the first Z warrior to challenge Cell, and after a bit of sparring as a warm up, Goku increases his Ki to maximum, which astounds the rest of the Z warriors. However, surprisingly, Goku's son Gohan expects more power from his father. After another bit of sparring, Cell forms a Kamehameha which threatened to destroy the Earth until Goku diverts it away from the Earth, and then returning to the ring using his Instant Transmission, only to surprise Cell with an attack. Cell attacks with a flurry of ki blasts. After much fighting, Goku reaches his limit and nearly falls from the ring. Cell, being tired of being confined to a small area, destroys the ring and announces that the entire Earth is the ring now. Goku, desperate to end the conflict, forms a Super Kamehameha while in the sky and makes Cell believe that he will fire the Chou Kamehameha and risk the existence of the Earth in order to defeat Cell. Goku teleports at the last moment in a close proximity of Cell, and shoots the Kamehameha, blowing off Cell's head and upper body. Although Cell was able to regenerate from the attack due to Piccolo's regenerative ability, it left both him and Goku significantly weakened. Goku, with his power dropping fast from the massive blast, announced that he was forfeiting the match, and then asks Gohan to take his place because he knew that Gohan had no trouble following the fight between him and Cell. As the Z warriors watch in awe, Goku reveals to everyone that Gohan is the only one able to beat Cell. Wanting the fight to be fair, he arrogantly gives Cell a Senzu Bean in order to recover from his energy loss.
Gohan challenged Cell, increasing his Ki to a point that rivaled even Goku's. Cell quickly launched a series of attacks on the young Super Saiyan, believing that he had finished Gohan when he had launched him through a mountain; he called on Goku to fight him once more. Goku points Gohan out to Cell, who emerges from the destroyed mountain practically unscathed. Gohan asks Cell to end the killing and tells him about his seemingly limitless dormant power that reveals itself whenever he goes berserk from rage. Cell is amused by this statement and decided to test it, attacking Gohan ruthlessly. When Cell fails to get Gohan angry, he then decides to provoke Gohan by attacking the Z warriors and quickly swipes the Senzu Beans. Goku had not taken one before, leaving him weak and vulnerable to Cell. Android 16 launches a surprise attack on Cell, grabbing him and attempting self destruction. Kuririn informs Android 16 that his self destruction device was removed by Bulma when she and her father repaired him. Cell escaped from No. 16's grasp and proceeded to obliterate him with a single blast.
Cell then returned his attention on Gohan's friends and, instead of wasting energy and risking defeat by fighting them directly, he creates seven miniature versions of himself, which he calls the Cell Jr.'s. The Cell Jr.s, each nearly as strong as Cell himself, attack the Z Fighters, effortlessly beating all but Vegeta, Trunks and Piccolo. At this point, it is revealed that even though 16 was destroyed by Cell, his head was still operational and happened to land where Mr. Satan and his pupils were hiding. Satan surprisingly proves his worth by tossing 16's head over to Gohan. 16 reassures Gohan that sometimes words cannot reach people like Cell, and tells him to release his power to protect the Earth. Cell then crushes the head of Android 16. Finally pushed beyond his emotional limit, Gohan's inner power awakens, transforming him into a Super Saiyan 2.
Gohan snatched the stolen Senzu Beans from Cell, who could not see how Gohan had managed to do so. Gohan then effortlessly annihilates the seven Cell Juniors It is then revealed that Goku was aware of Gohan's superior abilities all along, and Piccolo realizes that Goku used his own fight with Cell to give Gohan a preview of Cell's abilities; he never intended to beat Cell himself.
With the Z Warriors out of harm's way, Gohan returns the Senzu Beans and then utterly dominates the evil Android, severely injuring him with two simple hits, even after Cell increases his Ki to maximum. All of the Z Warriors (except Goku) were in awe of Gohan's awesome power, especially Vegeta, who had his pride hurt even more (it took him his entire life to become that strong while it only took Gohan a couple years). Attempting to make sense of this child's power that far exceeds his own, Cell fires a massive Kamehameha, knowing that if Gohan dodges it, it would destroy the Earth; to his surprise, Gohan easily counters it with a larger and stronger Kamehameha, pushing the blast back and destroying a significant portion of Cell's body. Goku tells Gohan to finish Cell, but Gohan, still in a rage from his awakened power leaves Cell to suffer. As Goku predicted, Cell regenerates and out of frustration and rage, transforms into a state that maximizes power, just like the USSJ. Trunks makes note that it was the same mistake that Cell mocked him for when he had fought him as a USSJ. Gohan beats Cell so hard that he is forced to regurgitate Android 18, reverting back to his Semi-Perfect form. Left with no other options, Cell initiates his Self-Destruction sequence, pumping his body up with enough Ki to destroy the Earth. Gohan is unable to do anything, and Goku steps in. He teleports Cell to King Kai's planet where Cell explodes, killing all life on the planet, including Goku.
After regeneration, Super Perfect Cell used Instant Transmission to return to Earth just moments after his seeming death, announcing his presence by killing Trunks with a single shot. Vegeta, shocked by the death of his son, launched a furious assault on Cell; it was in vain though, as Cell was unaffected by the attack and quickly slammed Vegeta aside. Cell then attempted to finish Vegeta off with a second ki blast, but Gohan intervened and took the full force of it himself, as a result, breaking his left arm. Cell wasted no time after this and fired a massive Kamehameha at Gohan, hoping to destroy Gohan along with the planet. Gohan, with spiritual assistance from Goku, launches a counter Kamehameha with his right arm. The two blasts meet in the middle and a power struggle between Gohan and Cell insues. Cell has the upper hand when the Z Fighters jump in, they're attacks don't effect Cell though, and he maintains his blast. The only attack that effects Cell at all is a powerful blast from Vegeta which throws Cell off and allows Gohan to destroy him.