The diameter of a circle is the distance across the circle at the widest part of the circle. The diameter is always equal to twice the radius of the circle, because the radius starts from the center of the circle and goes straight to the edge.
Learn by doing: measure the diameter of some circles in your house
Radius
Area of a Circle
More about Geometry
Bibliography and further reading about circles:
If a snooker ball has a diameter of 2.5 cms how many points of contact when colliding with a ball of exactly the same size are there?
That sounds like your homework? I’m sure you will learn more from doing your own homework than from having me tell you. Besides, just draw two circles next to each other to see the answer.