The square is a rectangle with four equal sides.
The sum of the interior angles of a square is 360 degrees.
Every corner is a right angle 90 degrees.
The area of a square is A=S²
The perimeter is P=4S
Moreover, one of the most important properties of the square is that its formed by two 45-45-90 triangles, therefore you can find its diagonal by multiplying a side by √2.
Let's do an example