The interface block is perhaps the least understood part of fortran syntax. It was introduced in Fortran 90 and serves some very important functions.
Coding in modern fortran, then its worth understanding interface Statement.
INTERFACE type FUNCTION name(arg-1, arg-2, ..., arg-n) type, INTENT(IN) :: arg-1 type, INTENT(IN) :: arg-2 .......... type, INTENT(IN) :: arg-n END FUNCTION name ....... other functions ....... END INTERFACE
The INTERFACE statement is the first statement in an interface block. The interface block is a powerful structure that was introduced in FORTRAN 90. When used, it gives a calling procedure the full knowledge of the types and characteristics of the dummy arguments that are used inside of the procedure that it references. This can be a very good thing as it provides a way to execute some safety checks when compiling the program. Because the main program knows what argument types should be sent to the referenced procedure, it can check to see whether or not this is the case. If not, the compiler will return an error message when you attempt to compile the program. Also with just a slight variation, it can then be used to create generic functions for use inside of a program unit.
Another additional function that the interface statement can perform is to provide a way to extend the performance of the assignment operator ( the equals sign ). Our general conception of this operator is that it just stores some value into a memory location associated with some variable. However when you think about it, the assignment operator will actually perform operations more complicated than that. For instance, when a real number is truncated to be stored as an integer or when spaces are added on to a character string to completely fill out the its allotted memory space. The interface statement can extend such abilities to instances that involve your own derived types.
The last function that the interface statement can preform is the extension of FORTRAN’s intrinsic operators ( +, -, .lt., .eq., etc. ) or the ability to create your own operators. For example, you could create your own operator called .pi. that you could use to transform angles input in degrees to their corresponding value in radians. Often times, things like that are just as easily carried out using a call to a function or a subroutine so don’t be concerned about the details. Just remember that the power exists to do such things in FORTRAN.
More info on interface in fortran at the following two links