Heuristics can be thought of rules. Typically heuristics are thought of as problem-specific strategies. Expert systems were an early form of AI that utilized rules-based decisions. In a game-playing context, heuristics can be pure strategies.
A heuristic function would be one that includes a some predefined decision rules. Russell and Norvig have a nice chapter on informed (heuristic) search strategies.
Heuristic functions are the most common form in which additional knowledge of the problem is imparted to the search algorithm.
Artificial Intelligence: A Modern Approach; 3.5 pdf