Circumcenter (C): Point in the plane equidistant from the three vertices of the triangle. i.e. C is on the perpendicular bisector of each side of the triangle. (any point equidistant from t two points on the triangle lies on the perpendicular bisector)
Circumcircle: The circumscribed circle of a triangle.
Note that the circumcenter lies at the center of the circumcircle.
If Geometers Sketchpad available: GSP Circumcenter and Circumcircle