The <iostream.h> library has been deprecated from the C++ standard specification, so it is recommended that you should use the standard-compliant <iostream> library instead of the older <iostream.h> library whenever you can.
There are some key differences between the these two librares:
First, the .h notation of standard header files was deprecated. Using deprecated features in new code is never a good idea.
Second, <iostream> contains a set of templatized I/O classes which support both narrow and wide characters. By contrast, <iostream.h> classes are confined to char exclusively.
Third, the C++ standard specification of iostream's interface was changed in many subtle aspects. Consequently, the interfaces and implementation of <iostream> differ from <iostream.h>.
Finally, <iostream> components are declared in namespace std whereas <iostream.h> components are declared in the global scope. Because of these substantial differences, you cannot mix the two libraries in one program.
As a rule, use <iostream> in new code and stick to <iostream.h> in legacy code that is incompatible with the new <iostream> library.