sara if this is an SLR i think you are talking about the iris and the reflecting lens. when the lens is mounted you can't see these things. try adjusting the f stops on your camera and you will notice the 3 pieces move in to make a circle. the smaller the f stop the bigger the ring, the bigger the f number the smaller the ring. if you chose f28 everything in your photo would be in focus, f2.8 and only a small area you select will be in focus. the mirror drops down when you press the shutter so that the film is exposed. try doing this without the lens on and by selecting a shutter speed of 1 second or b (which means as long as the shutter button is pressed the shutter will stay open) the 2 pieces of plastic and the rectangle are i think the shutter.... you will see this move if you do what i have said above. Most important is to keep fingers dirt and dust out of here... and don't try to glue it back together. post a picture of what you mean just so we're sure, but i think you are describing the normal innards of an SLR camera.
does it look like this? canon eos