Object - The thing that's emitting light (whether reflecting or producing, doesn't make a difference)
Image - A visual representation of an object; where light rays bounce of/are produced from the object, and they converge at a point (that's where the image is formed)
Center of curvature - If a spherical mirror were part of a sphere, then the center of the sphere would be center of curvature.
Focus - The point where the parallel light rays converge to form the image. If the aperture (see below) is too big, then the light rays will not converge at a single point, resulting in a distorted image
Pole - The Point inline with focus and center of curvature, at the center of the mirror
Focal length - The distance between the pole and the focus
Radius of curvature - The distance between the Pole and center of curvature, equal to the radius of the sphere. This distance is always , that is, two times the focal length. This means that the focus lies exactly between the center of curvature and Pole
Aperture - This is the diameter of the mirror itself. This defines how big the mirror is, how much light it can gather, and how much of the complete sphere it makes up.
Principal Axis - An imaginary line that goes through the Pole, Focus, and Center of Curvature