Who is Esme (Granny) Weatherwax from Lords and Ladies: A Novel of Discworld and what is their importance?

Esme (Granny) Weatherwax is the oldest and most powerful witch in Lancre. She is descirbed as tall and lean with long gray hair that she wears in a bun. Although Granny is sometimes hard to get along with, she is generally good-natured.... even if she is a prude.