Square and rhombus are two different shapes with some similarities between them. They both fall under the same family of quadrilaterals and parallelograms, which form the basis of the similarities between them. The similar features between them based on the fact that they are parallelograms are that; they both have four sides with the opposite sides equal.
As a matter of fact, all sides of both rhombus and square are equal. Talking about the quadrilateral part, this simply defines shapes that are having more than two sides, and of which both rhombus and square have four sides.
Though all the sides of both rhombus and square are equal, only the opposite sides of a rhombus are perpendicular, while all the sides of a square are perpendicular. In addition, all the four angles of a square are equal (i.e., 90 degrees), while only the two opposite angles in a rhombus are equal.