This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
It's mainly used in web/enterprise projects, to identify a user. For example, if you use a proper login system for a web application, the web container will make an instance of a Principal to identify the logged in user.