A circle is defined as a shape where all points are equidistant from the center.  The center is the point inside the circle and a circle is named by its center. The circle below is named Circle A.

Circle A

A radius is the distance from the center to any point on the circle.  If two circles are congruent, then the length of their radii are equal.  A diameter is a segment containing two points on the circle and the center of the circle.  A diameter is composed of two radii, therefore it is twice the length of the radius.

 Circle with radius and diameter

A compass is used to construct circles.  The distance from one point of the compass to the other point represents the radius of the circle.  Since a circle is all of the the points equidistant from a center point, it must encompass 360o.  In other words, to go around a circle exactly once, you would travel 360o.