Illustrated · 1 min
Rhombi are parallelograms with equal-length sides
Rhombi have four sides. The lines in each pair of opposite sides are parallel to each other, and all of a rhombus's sides are of equal length. This also means that the angles in opposite corners are always congruent.
Each of the blue shapes below is a rhombus. The red shape is not (but it is a parallelogram).

Squares are rhombi. All rhombi are parallelograms, but not all parallelograms are rhombi.
Area:
You can find the area of a rhombus by treating it like any other parallelogram.
Relationship to other quadrilaterals:
Rhombi are parallelograms with equal-length sides.
Squares are rhombi with perpendicular sides.