A rhombus a is a quadrilateral with 4 sides equal in length 
All squares are rhombuses 

Squares are quadrilaterals with 4 sides = in length AND 4 right angles 
Some rhombuses are squares (some not though)
because some rhombuses have angles not = to 90 degrees

So, all squares are rhombuses, but not all rhombuses are squares, Athera is correct.