A square has four sides of equal lengths. These congruent sides create four right, or 90 degree, angles. The opposite sides of the square are parallel. Parallel lines never intersect.
A rhombus also has four congruent sides, but the angles are acute and obtuse, not right. Two of the four angles in this figure are acute, or measure less than 90 degrees. The other two are obtuse, measuring more than 90 degrees. The two obtuse angles have the same measurement and the two acute angles have the same measurement. The opposite sides of the rhombus are parallel.
A rectangle has two sets of parallel, congruent sides and four right angles. A parallelogram also has two sets of parallel, congruent sides. However, the angles in a parallelogram are not right angles. Two of the angles are acute and two are obtuse. The acute angles are opposite each other and have the same measurement, and the obtuse angles are opposite each other and have the same measurement. The sum of all of the angles in both rectangles and parallelograms is 360 degrees.
Only two of the opposite sides of a trapezoid are parallel, and they are not congruent. The other two sides might or might not be of equal length. The trapezoid has one pair of opposite and congruent angles.