If I\'m writing abstract data types in C, are the functions written to perform actions on these data types and exposed in the interface (.h files) called functions,
Method is typically used to describe "functions" that are part of a class in OOP. Functions are just that - functions, not part of a class.
In the .h file I'd call them "function declarations" and in the .c I'd call them "function implementations" ... I'm no expert though, just my .02.