Android 17 was created by Dr. Gero as a weapon of the Red Ribbon Army. His main purpose: Kill Goku and complete Dr. Gero's revenge. Along with his counterpart, Android 18, they were originally humans. When Gero, now an android himself, was taken by surprise by Vegeta's Super Saiyan transformation, he woke 17 early.
Once revived, 17 quickly killed Gero and with his sister woke the experimental Android 16. Together they all sought Goku to complete their mission. On their way, 17 tried to get Piccolo to reveal where Goku was, as since he was in bed with the heart virus, never appeared. 17 fought with Piccolo, who was confident now that he merged with Kami and become a Super Namekian, before Cell appeared, and absorbed 17 so he could reach his Perfect Form. After absorbing 17, Cell gained a tremendous spike in power, and gained the strength to defeat 16.
After Cell's defeat, the Dragonballs were used to resurrect all those that Cell had killed, including 17. He was later seen leading a normal life and donating his energy to help Goku defeat Majin Buu.
In an alternate future, 17 and 18 easily killed all of the Z-Fighters, leaving only Gohan and Trunks. This version was far more evil and ruthless. Gohan was killed in battle and Trunks came back in time to warn those from the past of the androids awakening. Trunks helped in the past and returned to the future far more powerful, easily killing both androids.
Dragon Ball GT
In Dragon Ball GT Android 17 was used by Gero and Dr.Myuu to merge with another Android 17 designed by them in hell. The fusion resulted in the creation of a new villain Super Android 17, while Gero believed he was controlling the Android but Dr.Myuu was actually the one giving orders Gero was Killed by Super Android 17 who also killed Dr.Myuu later after being confronted by Android 18 about taking orders from others and not being able to make his own decisions. He was defeated by Goku and Android 18 working together, 18 distracted him while Goku used Dragon Fist attack on 17 to finish him.
Dragon Ball Super
17 works for a wildlife reserve and is battling with some poachers and is confronted by Goku who offers to help but with 17 replying not to kill the poachers. Eventually, the poachers are retreat. Goku, wanting to see how strong 17 is, offers to fight with 17 of which he accepts.
Goku transforms into a Super Saiyan and clashes with 17 but is overwhelmed. Goku decides to transform into a Super Saiyan Blue. They fought an even match until 17 decided to stop the match due to the dolphins breaching. 17 and Goku later have a conversation with Goku asking 17 if he once to join the Universe 7 team at which 17 refuses but later joins after Goku helps 17 defeat an alien boss.
In the Universe 7 saga, 17 along with his Universe 7 teammates battle the opposing teams with 17 battling the Pride Troopers and beating them.
Powers and Abilities
17 like his sister 18 was experimented upon by Dr. Gero and had cybernetic graphs implanted in his body to exceed any of the Z-Fighters at the time. Android 17 has strength, speed, enhanced durability, and is noted to be physically stronger than his sister. Also like his sister 17 has an internal energy source that never runs out giving him unlimited energy and stamina in a fight. At activation 17 had enough power to dominate over Vegeta, Future Trunks, Piccolo, and Tien Shinhan and then to match-up to Piccolo after he fused with Kami. While during the Cell Saga 17's power was quickly eclipsed by most of the Z-Fighters and Cell himself, 17 was still considered quite powerful. By the time of the Universe Survival arc 17 had been training in secret for 10 years and now can match up to a suppressed SS Blue Goku though 17 was holding back as well. This means 17 now has power in the same league as SS Bue Goku, SS Blue Vegeta, Golden Frieza, Goku Black, and Mystic Gohan.
Android Barrier - 17 creates a blue powerful barrier made out off energy which he uses to protect himself from attacks.
Accel Dance - This is a technique where 17 and his sister, 18, attack an enemy with combos and finishes the attack with a barrage of yellow ki blasts to a downed opponent.