Supergirl is Superman’s cousin. Her name is Kara Zor-El. She has toe different origins. One is she is the last surviving member of Argo City who is sent to Earth by her father to be raised by her cousin Kal-El(Superman). She is younger than her cousin in this origin. In her other origin, she is born before Superman, and is placed into suspended animation to accompany him to Earth. Her spaceship malfunctions, and she reaches Earth much later than Superman, and due to her being in suspended animation, she appears much younger than Superman.
Supergirl was created as a younger female version of Superman. She has all of his strengths and weaknesses. The biggest difference is that she doesn’t have Superman’s affinity towards the human race, and sees them as weaklings who are below her. She is really quick to act, and is short tempered. She was killed during the Crisis in Infinite Earths storyline to reestablish Superman as the sole surviving Kryptonian. She was reintroduced to the DC universe in 2004. No one stays dead in comics(except Uncle Ben). When they brought Supergirl back, that is when they changed her origin.
Supergirl had her own movie in 1984. It sucked. It was just a rush job. Supergirl was on Smallvillefor two seasons. Supergirl was also on the Superman: The Animated Series and Justice League Unlimited.
Supergirl really wasn’t in comics when I started to read comics, but I always knew about her. I used to think she watered down Superman, but over the years I have started to think the opposite. She has helped add depth to Superman, showing him in a big brother/tutor mode.
You can learn more about Supergirl here.