Diameter is the distance across a circle through its centre
Radius (radii is the plural) is the distance from the centre of a circle to its outer edge
We can do the formula 2R=D this means that 2 radii are needed to make a diameter of a circle.
1/2 x D = R
D/2 = R
Here's a video....
This has some extra things but it's still helpful.
More Help? Then Check this out. (it has some extra stuff but it still helps)