In utilitarianism, what differentiates 'act' utilitarianism from 'rule' utilitarianism?

The core difference between act utilitarianism and rule utilitarianism lies in how they apply the principle of maximizing overall happiness. Act utilitarianism applies the utility principle – that actions are right as they tend to promote happiness, wrong as they tend to produce the reverse of happiness – directly to individual actions. Each individual action is evaluated separately to determine if it produces the greatest good for the greatest number. In any given situation, the act that results in the most overall happiness is the one that should be chosen, regardless of any pre-established rules. Rule utilitarianism, on the other hand, applies the utility principle to rules. Instead of evaluating each action individually, rule utilitarianism identifies general rules that, if consistently followed, would tend to maximize overall happiness. The morally right action is then the one that conforms to these rules, even if in a particular instance, following the rule does not produce the most happiness. For example, an act utilitarian might justify stealing food to feed a starving family if the happiness gained by the family outweighs the unhappiness caused to the store owner. A rule utilitarian, however, might argue that stealing should never be a general rule because it would undermine property rights and lead to widespread unhappiness, and therefore stealing is wrong even in this specific case.