Reports from reviewers who have already seen Disney Pixar's film Brave, tell us that the witch from the movie is not highly involved in the plot. The witch's character and looks are modeled after Asian folklore, rather than the black pointed hat with a broom witch, from western culuture that most come to expect.
Similar to the witch in Snow White, she is a wrinkled old women who seems as if she couldn't cause any harm. While she seems like she couldn't cause any harm she adds a twist to the plot and a much needed injection of humor.
Disney has released a short video clip showing the witch in action. In my opinion the witch is hard to understand, but at the same time she is a much needed character in the cast of Brave.