Appearance:
Android 17 is the first artificial human designed by Dr. Gero to be created by fitting a Human Being with cybernetics. As a mortal man, he and his unnamed sister (the eventual Android 18) were runaways whom Gero kidnapped and conducted experiments on, resulting in their transformation into cyborgs. Gero's blueprints for Android 17 in Funimation's dab of the Imperfect cell saga suggest otherwise however, instead citing that 17 and 18 "are made up of organic substances padded after human cells." On the other hand, aside from nothing there suggesting that the human cells cited couldn't be their own original human cells, every other account of the series, including biography sections within video games and a piece in a later saga within the same dub (albeit with the statement specifically about 18), agree with the former.
Abilities:
Android 17 is one of the most powerful fighters in the world, due to the cybernetic enhancements Dr. Gero gave him, including an unlimited supply of energy, allowing him to have endless stamina and not tire out in a fight. He was powerful enough to be an even match for Piccolo after he fused with Kami (however, because of his endless power, Piccolo would eventually tire, allowing 17 to win). Considering his sister, Android 18, was so easily able to defeat Super Saiyan Vegeta, 17 is supposedly programmed to be a bit stronger than his sister, meaning he can easily overwhelm an average Super Saiyan, but anything above that power might prove a challenge to him. Although he is an android, he can still improve his strength through training, like the other Z Fighters; this is shown when he fights Vegeta in Dragon Ball GT, and though Vegeta has trained much through the years, #17 proves to be an even match for him. Besides his power, Gero's android technology effectively makes 17 immortal in the sense that he can't age
