The only difference between them is that a square is a rectangle with equal sides. Both have 4 90 degree (right) angles. A triangle has 3 sides and 3 angles. The angles of a triangle always add up to 180 degrees.
Keeping this in view, what are all the properties of a square?
What is a square person?
Square used as slang may mean many things when referring to a person or in common language. It is often used to speak of a person who is regarded as dull, rigidly conventional, and out of touch with current trends. An agreement that is equitable on all sides is a "square deal".
Why square is a special rectangle?
So a square is a special kind of rectangle, it is one where all the sides have the same length. Thus every square is a rectangle because it is a quadrilateral with all four angles right angles. However not every rectangle is a square, to be a square its sides must have the same length.