The key difference between square and rhombus is square has all its angle equal to 90 degrees, but rhombus does not have. But both the shapes have all their sides as equal. See the below figure to know the difference.
Definition of Square and Rhombus
Square: A square is a foursided polygon which has all its sides equal in length, and all the angles are equal to 90 degrees. The length of the diagonals is also equal.
Rhombus: A rhombus is also a foursided polygon, and all the sides are equal in length. The opposite angles of a rhombus are equal, and the diagonals bisect each other at 90 degrees.
What is the difference between a Square and a Rhombus?
Square and Rhombus Differences 


Square  Rhombus 
The diagonal lengths of a square are of the same measure.  The diagonal lengths of a rhombus are of different measure. 
All the interior angles of square measures 90 degrees (i.e., right angle)  In a rhombus, the opposite angles are of the same measure. 
Square and Rhombus Similarities 

A Square is a quadrilateral.  A Rhombus is also a quadrilateral. 
All sides of a square are equal in length.  Similar to the square, all the sides of a rhombus are also of equal length. 
Opposite sides are parallel to each other.  Opposite sides of a rhombus are also parallel to each other. 
Diagonals are perpendicular to each other.  Similar to that of a square, the diagonals are of a rhombus are also perpendicular to each other. 
These are some of the similarities and differences between rhombus and square. Check out more similar articles from the links given below:
I liked the differences of the square and the rhombus and how you put it in perspective.