Witch is a term for creatures born after a Magical Girl falls into despair and their Soul Gem turns into a Grief Seed as a result. They hide in mazes called Labyrinths and control minons called Familiars. They are very dangerous to humans, hence why Magical Girls fight them. And the cycle repeats once that Magical Girl becomes a Witch, trying to killing one person for each one she saved. After Madoka Kaname recreates the universe however, they are erased from existence completely and have been replaced by Demons.