Ellen White was a young woman around during the formation of the Seventh-day Adventist church. Over a period of 53 years, she had 2000 messages, or visions, from God that were the basis of thousands of pages of written material. Although many have called her a prophet, she never claimed the title herself. Subjects of her numerous books range from history and religion to nutrition (about which she held a very holistic approach decades ahead of her time). Seventh-day Adventists believe her writings are works inspired by God, but that they should be judged by the standards, instructions, and principles of the Bible.
More info at: http://www.whiteestate.org/about/egwbio.asp#who