A parallelogram is defined as a four sided figure that has two sets of opposite sides that are parallel to each other. So rectangles and squares are special types of parallelograms.

There are two ways I have seen trapezoid defined. One way merely calls it a four sided figure with two parallel sides. The other way defines a trapezoid as a figure that has only two parallel sides.

So a rectangle is definitely a parallelogram, and if you were to go with the first definition of trapezoid, a rectangle could be a special form of a trapezoid, though if you were to go with the second definition, then a rectangle would not also not be a trapezoid.

The trapezoid controversy is the result of a dispute between elementary geometry teachers (who think of a trapezoid as having exactly one set of parallel sides) and the mathematicians (who think of a trapezoid as having AT LEAST one set of parallel sides).

IMO the second definition is the better one, as a lot of higher math will break down without it, but we're still at an math-cultural impasse. I don't think the GMAT will take a side in the debate, so you shouldn't see any questions requiring you to have an opinion on whether a square is a trapezoid.