A rectangle is a noun referring to a specific type of shape, while oblong is an adjective used to describe shapes or forms. A rectangle can be characterized as oblong.
A rectangle is a parallelogram with four right angles, each of which measures 90 degrees. Oblong is defined as elongated, usually from the square or circular form. When referring to a rectangle, oblong can be described as when one set of parallel sides is longer than the other set. An elongated or oblong square can be classified as a rectangle, while an elongated or oblong circle can be classified as an oval.