This is a curved mirror as it might appear from the side. This could represent either a concave or convex mirror. The difference is simply which side is the real side. With a mirror the side of the mirror you are on is the real side while the "back" side is the virtual side.
The optical axis cannot be sen but is a useful line which passes through the center of the mirror.
The apex is simply the point where the optical  axis intersects the mirror.
C is the center of curvature of the mirror.
The distance from C to the apex would be the radius of the full circle the mirror is a section of. This will be important when drawing normals.
Which type of curved mirror is shown in this diagram?