This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
std::string is the string class from the standard C++ library. String is some other string class from some other library. It's hard to say from which library, because there are many different libraries that have their own class called String.