He is the Emperor of the Universe and at the time was considered the most powerful being in the universe. He is considered a prodigy and thus, his power was given to him at birth and he never trained a day in his life until the Resurrection F in Dragon Ball Super.
In addition to his title as Emperor of the Universe, he also has planets wiped out and sold to the highest bidder. For this, he is greatly feared by many in the Universe with Beerus one of the few at the time Frieza feared.
Frieza also fear a prophecy that a Super Saiyan would defeat him. To prevent this, he wiped out Planet Vegeta, taking the prince with him along with two others. At the same time, a baby Saiyan reached Earth, Son Goku. Goku would become Frieza's archenemy and the Super Saiyan he feared so much.
Frieza is considered one of anime's most evil villains and has multiple forms.