What is the difference between a commissary and a px?

Excuse me, could you please clarify the distinction between a commissary and a PX? As I understand, both of them are associated with providing goods and services to military personnel, but I'm not quite sure how they differ in terms of their operations, offerings, or accessibility. Could you elaborate on the key differences between the two, and perhaps provide some examples to help me understand them better? Thank you in advance for your assistance.