In this video the instructor shows you the diameter, radius and also how to find the circumference of a circle using a cookie as an example. When you draw a line against the cookie passing through its center and touching its boundaries, it is a diameter of the circle. Radius of a circle is half the diameter of the circle. The length of the circle along the border of the circle is called circumference of the circle. The formula to compute the circumference of a circle is 2 * Pi * r, where r is the radius of the circle which is half the diameter and value of Pi is 22/7. In the video the instructor demonstrates these concepts using a large cookie. This video explains the basic concepts of a circle like diameter, radius and how to calculate its circumference.