What is the difference between a rhombus and a square?

That a rhombus is that a square, by definition, is an equilateral parallelogram. That is, each of the four sides are equal in length and are also pairwise parallel. The difference, in principle, is only in one thing: a square has straight angles, but a rhombus can have different angles – opposite angles are equal in pairs, based on the parallelism of the sides from which the angles are formed, in total give 360 degrees.
So it turns out that the square is a special case of the rhombus.