You should understand the following terms clearly,
Center-----A point inside the circle. All points on the circle are equidistant (same distance) from the center point.
Radius-----The radius is the distance from the center to any point on the circle. It is half the diameter.
Diameter-----The distance across the circle. The length of any chord passing through the center. It is twice the radius.
Circumference-----The circumference is the distance around the circle.
Area-----The region enclosed by the circle.
Chord-----A line segment linking any two points on a circle.
Tangent-----A line passing a circle and touching it at just one point
