In Dragonball Z, Dr. Gero creates android 18 in order to take out revenge on Goku. Android 18 is his 18th cyborg creation, hence her name, ultimately she will become Krillin's wife and the mother of their daughter Marron.
She actually began life as a human before she was bio-mechanically altered to become one of Dragonball Z's butt-kicking cyborg fighters. Android 18's appearance is somewhat misleading, since she seems to be a petite and beautiful blonde girl, but is in all actuality a talented and deadly fighter. Dragonball Z heros Goku and Android 18 have several vicious fights especially because she is designed as a direct counter-weapon to Goku's fighting styles.
Ironically, because android 18's memory had not been totally wiped out by Dr. Gerro before being reactivated as a weapon, she and her sister, android 17, rebel and attack Dr. Gero, killing him. They then begin a reign of terror and proceed to kill all of the Dragonball Z 'team Z' warriors excepting Gohan. Gohan escapes this fate by becoming a super-saiyan.
Goku is one of the most recognizable characters of the entire Dragonball Z world, appearing alternatively as a smaller younger and less powerful version as well as a super-saiyan with ultimate fighting abilities.
Goku and android 18 fight but unfortunately for Goku he cannot quite match up to the power of this precisely designed and maximally outfitted fighting cyborg. Born and programmed to hunt down and kill Goku in order to carry out Dr. Gero's vendetta, ultimately the blonde femma fatale succeeds in this mission. However, it is interesting to note that due to the memory wipe Dr. Gerro performed on her not being total and flawless, she still retains some of her memory and her rebellious spirit. In fact, as a cyborg she is even capable of feeling pain even though her bio-mechanical alterations make her more resistant to damage than purely organic lifeforms and combatants like Goku. This makes her one of the most enduring fighters in the world of Dragonball Z.
The truly remarkable and interesting thing about android 18's character is exactly how powerful she was and well-suited to defeating Goku and his comrades upon her debut. None of the regular warriors were able to hold a candle to this finely tuned instrument of destruction and combat, making her one of the more formidable opponents to challenge Dragonball Z's 'Z warriors' in memory.
She was able to resist even their most powerful attacks and absorb much higher degrees of damage and punishment than any of her fleshy Dragonball Z adversaries could hope to. Meanwhile, she demonstrated her superhuman strength by dislocating Goku's arm with a single robotically-enhanced kick! Definitely not a force to be underestimaed, Dragonball Z's android 18 made her presence and threat known. Android 18 and Goku were two of the more memorable match-ups in the dragon ball z series because of the intense and almost desperate combat moves which Goku attempted, yet ultimately failed, to defeat her with.