A square is always a rhombus; it is a special kind of rhombus where all four corners are right angles. A rhombus is a two-dimensional geometric shape with four equal-length, straight sides.
A rhombus is sometimes called a diamond, because it makes a traditional diamond shape. Rhombuses are a type of parallelogram, which means their opposite sides are always parallel, and their opposite angles are always equal. A square is the only type of rhombus where all four angles are equal, and the only rhombus that also meets the definition of a rectangle (all angles are 90 degrees). Any four-sided geometric shape can also be called a quadrilateral.