A square is always a rhombus because it satisfies the definition of a rhombus, which is any figure with four equal sides, equal opposite angles and two pairs of parallel lines. A rhombus can sometimes be a square.
To be considered a square, a shape must have two pairs of parallel lines, four right angles and four equal sides. A rhombus does not require four equal angles to be considered a rhombus, but only rhombuses that have four equal 90 degree angles can be considered squares.
Rhombuses and squares are both types of parallelograms. A parallelogram is defined as any figure with two sets of parallel lines.