The sides’ length and number of pairs of parallel sides. See explanation.
A trapezoid is a quadrilateral with at least one pair of parallel sides (called bases), while a rhombus must have two pairs of parallel sides (it is a special case of a parallelogram).
The second difference is that the sides of a rhombus are all equal, while a trapezoid may have all 4 sides of a different length.
The other difference is the angles: a rhombus has (like all parallelograms) two pairs of equal angles, while there are no limitations to angles of a trapezoid (of course there are limitations which apply to all like: the sum of all angles are 360 degrees).