The Evil Queen, whose real name is Queen Grimhilde, is the main villain of Snow White, her most famous incarnation is in Disney's classic animated feature but the origin of the story goes as far back as folklore itself - being a popular fairytale. Disney's film had a tremendous impact on how people see the Evil Queen as a villain and introduced the concept of her Magic Mirror and her ultimate transformation from an attractive but vain woman to a hideous old hag.
The Evil Queen married an unnamed King who died. She despised her step-daughter - Snow White - due to her beauty and forced her to dress in rags while treating her as a servant rather than a daughter: throughout the story it was made clear the Evil Queen's motive was her vanity, she simply couldn't stand the idea of someone being more beautiful than herself and was cruel enough to take drastic steps to ensure this could never happen.
This was shown when, upon asking her Magic Mirror who was the fairest in the land, the Evil Queen was informed that it was Snow White - she grew angered and ordered her Huntsman to take Snow White to a field and kill her: wishing to have proof of the deed she gave the Huntsman a box and ordered him to bring back Snow White's heart once his task was done.
However the Evil Queen didn't count on the fact her Huntsman was more merciful than herself and he spared Snow White's life, telling her to flee and never return - he then killed an animal from the forest and gave its heart to the Evil Queen: tricking her into believing Snow White was dead.
In time the Evil Queen became aware of Snow White's existence when her Magic Mirror once again informed her that Snow White was the fairest in the land and that she was alive and being taken care of by a group of seven dwarves in the forest. Driven mad with rage the Evil Queen utilized her knowledge of the dark arts to transform her once noble (albeit wicked) form into that of a grotesque and frightening hag - she then created a poisoned apple and made her way to the dwarves' cottage. The Evil Queen waited until the dwarves had gone to the mines to work until she arrived, pretending to be a harmless old woman and offered Snow White the poisoned apple - Snow White innocently took the apple and collapsed to the ground as the Evil Queen cackled madly and left the scene.
However the woodland animals were quick to inform the dwarves of what transpired and they raced into action - the dwarves pursued the Evil Queen up onto a mountain where she attempted to roll a boulder down to crush her pursuers but slipped in the process, causing her to fall to her death as the boulder fell onto her.
The dwarves returned to the cottage and believed Snow White was dead yet couldn't bare to bury her so encased her in a glass coffin - she would later be awoken from the Evil Queen's curse by the kiss of her one true love: the Prince, and the two would proceed to live "Happily Ever After".