C++ Programming/Standard Headers

Standard Headers
and the 

Everything inside C++'s standard library is kept in the std:: namespace.

Old compilers may include headers with a .h suffix (e.g. the non-standard  vs. the standard  ) instead of the standard headers. These names were common before the standardization of C++ and some compilers still include these headers for backwards compatibility. Rather than using the std:: namespace, these older headers pollute the global namespace and may otherwise only implement the standard in a limited way.

Some vendors use the SGI STL headers. This was the first implementation of the standard template library.