Irregular shapes are those with sides of different lengths and angles that are unequal. Regular shapes have sides with equal lengths and have angles of the same size.
Shapes like squares and equilateral triangles are considered regular shapes because they have predictably identical sides and angles. If a person were to take an equilateral triangle and make each of the sides different lengths and change the size of each angle, the regular-shaped triangle would turn into an irregular shape. Irregular shapes do not have to look messy or weird, though; a shape that resembles a rectangle with a chunk cut out of it can be considered irregular, even though it is composed of straight lines and has a neat and orderly appearance.