There are Prolog implementations that are radically different, with different syntax and different semantics (e.g. Visual Prolog) and sub-communities have developed around different implementations.
Code that strictly conforms to the ISO-Prolog core language is portable across ISO-compliant implementations. However, the ISO standard for modules was never accepted by most Prolog implementors.
Factors that can adversely affect portability include: use of bounded vs. unbounded integer arithmetic, additional types such as string objects, advanced numeric types (rationals, complex), feature extensions such as Unicode, threads, and tabling. Use of libraries unavailable in other implementations and library organisation:
Currently, the way predicates are spread over the libraries and system built-ins differs enormously. [...] Fortunately, there are only few cases where we find predicates with the same name but different semantics (e.g. delete/3)
^ abC/Java interface can also be used for graphics and OS control.
^B. Demoen, and P. Nguyen, About unnecessary performance differences between Prolog implementations, Proceedings of the Colloquium on Implementation of Constraint and Logic Programming Systems (CICLOPS 2001)