A rhombus is like a parallelogram, in that both pairs of opposite sides are parallel

But it goes one stage further. Instead of opposite sides being equal, with a rhombus all four sides are equal.

Of course if all its angles are right angles as well as all its sides being equal, the shape is a square. So you can think of a rhombus as a square that has got knocked over a little. All four sides are equal, but its angles are greater or less than 90°.

You also can note that, like a parallelogram, its opposite corner angles are equal to each other.