eyeseeyou:
A rhombus is simply a quadrilateral with all 4 sides equal. That is all you need to define a rhombus.
A square is a rhombus - but not all rhombuses are squares.
From the simple definition above, it can be easily shown that a rhombus is also a parallelogram - therefore it inherits all the properties of a parallelogram, such as: opposite sides are parallel, opposite angles are equal, diagonals bisect each other.
In addition, the rhombus has the additional properties: the diagonals bisect each other at right angles, and the 2 diagonals cut up the rhombus into 4 congruent right-angled triangles and each diagonal divides the rhombus into 2 congruent isosceles triangles.