Ah, terminology questions.  Fun.

Let's take them one at a time.

1. 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.
2. 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.)
3. 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!