Scar is Mufasa's brother and Simba's uncle. His birth name was Taka; which is never said in the movie, but all the true fans know. He was "denied" the throne and can't forget it. Full of vengance, he sets out to de-throne his brother and wipe the young prince from the picture.
Simba survives Scar's plot (barely) and returnes to overthrough this totalitarian-ruler of an uncle. Scar almost ends it all again, but past memories and anger give Simba the final kick in the matter.
