Difference between radius of curvature or …

Radius of curvature refers to the radius of the sphere of which mirror is a  part. It usually consists the length between the pole of mirror and centre of  curvature.
Principal Axis refers to an imaginary straight line passing through  pole, focal point and centre of curvature.
Radius of a curvature is a part of Principal Axis while principal axis is a whole.

1...Radius fo curvature is distance between pole and centre of curvature while principal axis is an imaginary line joining pole focus and center of curvature of a mirror (2).....radius of curvature is of a definite measurement while principal axis is of indifinite length{however in case of numericals we never consider length of principal axis important}(3)......radius of curvature lies in front of a concave mirror and behind convex mirror while principal axis can be extended even infront and behind of mirrors(any) (4)......radius of curvature have got special sign conventions while principal axis do not (5)radius of curvature in case of convex miror is virtual while in case of concave mirror is real but principal axis is nither virtual nor real( but we take them real or virtual in some special cases) (6)......in case of convex mirror radius of curvature is positive and in concave mirror redius of curvature is negative but principal axis is never taken positive or negative(7)......radius of curvature is shorter in length as compared to principal axis ------------------------The most important point------------------radius of curvature is a part of principal axis