They are all connected. Diameter is just how wide it is by drawling a straight line that passes through the center from one side to the opposite end. It does not matter where you start the line because the diameter does not change. It will always be the biggest width possible.
Radius is half the Diameter. Example: Diameter is 10 feet, so Radius is just 10/2 = 5 feet. It is the distance from the center of a circle to the outside of it.
Circumference is a measure of the outside circle length. Imagine you are taking a walk around the circle on the edge. When you get back to your starting point that is the circumference.
Now, there is a special number called pi that links these things all together. pi is the ratio of the circumference / diameter. That number is always constant and about 3.14 (actual number goes on forever, but that is more complicated and 3.14 is good enough most of the time).
If you only know the radius, I can get the diameter by doubling it.
Radius = 5, Diameter = 10
Since pi = Circumference / Diameter, I can calculate the Circumference just by taking the Diameter times pi. Example: Diameter = 10 ft, so Circumference is 10 ft X 3.14 = 31.4 ft. Area is pi X radius squared, but that is more advanced.
