Ah, terminology questions. Fun.
Let's take them one at a time.
- Radius of Curvature -- This has to do with how "curved" the mirror is. Is it almost flat, or does it form an elongated "U" shape? This will have a lot to do with the focusing power of the concave mirror.
- Principal Focus -- This is the point on the mirror that the reflected light is aiming toward (in a concave mirror) or aiming away from (in a convex mirror.)
- Aperture -- This is how much light the mirror is able to focus. It is a measure of how much light can hit the mirror. It is both a product of how big the mirror itself is, and how much light is allowed to hit it (as if the mirror had a black piece of paper over it with a tiny hole, for example.)
Hope these examples help clear up any questions you had about these terms!