A rectangle is a specific type of oblong shape. "Oblong" can also refer to other elongated shapes, such as ovals, certain trapezoids and parallelograms. Rectangles are usually referred to as rectangles, while "oblong" is an adjective that describes rectangles and other shapes.
A rectangle is defined as a quadrilateral having two sets of parallel sides and four 90-degree interior angles. This is a very specific definition. A rectangle is a specific type of square as well as a specific oblong shape.
On the other hand, "oblong" is a general word that applies to a number of different shapes. Not all oblongs are rectangles, just as not all squares are rectangles. Oblong shapes include ovals and other types of parallelograms. In addition, the word "oblong" is not only a math term. It can also refer to objects that are longer than they are broad.
Any elongated shape or object may be called oblong, though oblong is not the most specific definition. Rectangles, however, are strictly defined and must conform to their own properties of right angles and congruent sides. The difference between oblongs and rectangles is a matter of the preciseness of each definition. "Rectangle" is the more precise of the two terms.