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.
Thread safe classes are classes that can have multiple callers using the same instance at the same time. Can you elaborate a bit on what you are confused about? This will help get a more specific answer.