What's the difference between <iostream> and<iostream.h> ?

Answer 1

For most compilers (unless it's really old) <iostream.h> is deprecated use <iostream>

Answer 2

iostream and iostream.h are different header files, in fact. iostream.h is not part of the standard library while iostream is. Somewhere along the line when iostream was added, it was decided not to remove the older iostream.h header for backwards compatibility purposes.