All squares are rectangles, but not all rectangles are squares. The definition of a rectangle is a shape that has four sides and four 90-degree internal angles, which is also true of all squares.
A square and rectangle are both quadrilaterals, which are four-sided shapes. A square is a special kind of rectangle because all of its sides are equal. Due to all angles in both squares and rectangles being 90 degrees, both types of shapes share the property that opposite sides are parallel to each other and connecting sides are perpendicular. A square can also be considered a rhombus, which is a quadrilateral defined by having four sides of equal length and equal opposite angles.