In the United Kingdom they are (normally a "Command paper") issued by the government and laying out its policy, or proposed policy, on a topic of current concern. Although a white paper may occasion consultation as to the details of new legislation, it does signify a clear intention on the part of a government to pass new law. This is a contrast with green papers, which are issued less frequently, are more open-ended and may merely propose a strategy to be implemented in the details of other legislation.
Famous white papers
Famous green papers