What Are Rectangles?

Rectangles are parallelograms with perpendicular sides

Rectangles have four straight sides. Each pair of opposite sides is parallel, and adjacent sides are perpendicular. This means that every angle in a rectangle is a right ($90^\circ$) angle.

All the blue shapes in the image below are rectangles. The red shape is not a rectangle (though it is a parallelogram).

Area:

You can find the area of a rectangle by multiplying $\text{length}\times\text{height}$, where $\text{length}$ is the length of one of the sides and $\text{height}$ is the length an adjacent side.

Relationship to other quadrilaterals:

Rectangles are parallelograms with perpendicular sides.

Squares are rectangles with equal length sides.