A closed shape with only straight edges is called a polygon. A polygon with all sides and all angles equal is called regular.
A polygon with 4 sides is called a quadrilateral. A regular polygon with 4 sides is called a square.
A quadrilateral with four right angles but just opposite parallel sides of equal length, is called a rectangle.
The area of a square of side x=$x^2$ sq. units. Perimeter=$4*x$ units.
Area of a rectangle with adjacent sides a and b=$ab$ sq. units. Perimeter=$2*(a+b)$ units.

The above graphic illustrates how to compute the total interior angle measure of a polygon, and the measure of each angle if it is regular.