Angles and sides are the main differences! In a rectangle, all angles will be 90 degrees. In a rhombus, opposite angles will always be congruent. And, all sides in a rhombus are congruent, while in a rectangle, opposite sides are congruent.
An easy way to think of it is that, if a rhombus and a rectangle had a baby, it would be a square! A square gets its 90 degree angles from the rectangle, and the congruent sides from the rhombus. Hope that helped!