Android 18 was an Android who was modified and created by Dr. Gero to destroy Earth. She used to work alongside Android 16 and Android 17. She was an incredibly powerful Android who was strong enough to fight Saiyans like Vegeta.
When she first appeared, Android 18 was vicious and heartless. She was described as a being that was out for destruction and chaos. When she heard of Cell's revival, she was fearful of what Cell was capable of doing and was afraid of getting assimilated by him. She was willing to assist the Z Fighters against Cell. She also likes Krillin. Even after her revival, Android 18 became friends with the Z Fighters and their family and friends.
When she was created and modified by Dr. Gero, it was stated that she possessed the power along with Android 16 and Android 17 to destroy the world. Android 18 along with other Androids have a massive supply of power behind them. Like all Androids, Android 18 can fly, shoot energy blasts, and posses superhuman strength. In her fight against Vegeta, she also posses superhuman reflexes. While fighting Vegeta, her superhuman strength was great enough to break the Saiyan's arm with a single kick which caused him to scream in pain. This was a testament to her powers as she was able to fight in a one-on-one fight with a Saiyan without any resistance.