The radius is the line from any point on the circumference connected to the center of the circle. It is the distances that all points on the circle are from the center.
A chord is a line connecting any two points on the circumference to one another.
The diameter is any two points on the circumference that when connect intersect with the center, it is the magnitude of the longest chord possible and is twice the radius.
The center is that point from which all points on the circumference are an equal distance away from.