Squares and rectangles both are a type of quadrilateral, both have their four interior angles equal to 90 degrees and opposite sides parallel to each other.
The similarities between a rectangle and a square as:
All 4 angles are 90 deg each.
Opposite sides are parallel and equal.
The diagonals bisect each other.
Circles can circumscribe both figures.
The diagonals form the diameter of the circumscribing circle.
The square root of the sum of the squares of adjacent sides is the diagonal.
The area is the product of adjacent sides.
The diagonals bisect the figure into 2 congruent triangles.
Join the midpoints of opposite sides and you get 4 similar figures of 1/4 the area of the original figure.
Join the midpoints of 4 sides in order and you get a rhombus, whose area is half that of the original figure.