iostream and iostream.h

<iostream> is encapsulated in namespace std, but <iostream.h> doesn't.

If you use #include <iostream> , you have to using namespace std.

If you use #include <iostream.h> , you won't use any namespace.

Reserving the iostream.h to compatible with previous source code.

Reference:

http://www.cplusplus.com/forum/beginner/12291/
http://stackoverflow.com/questions/214230/iostream-vs-iostream-h-vs-iostream-h