What is the rule for a perfect square?

Can you clarify for me what constitutes a rule for a perfect square? Is it a mathematical formula or a set of guidelines that determine if a number is a perfect square? I'm interested in understanding the criteria or principles that govern the classification of a number as a perfect square, as well as the logic behind it. Could you provide a concise explanation of the rule and maybe even give an example or two to illustrate the concept?