1 ACCEPTED SOLUTION
" One would want to think that the 40mm is covered within the 18-55mm lens ..."
And it is. 
All the rest of this is nonsense and meaningless. They are just numbers to help label something.
The difference lies in the fact one is a zoom and the other is a prime. And than only because the f ratio is different. Again a number to tell you how much light will be transmitted to your camera. All you need to remember is, the smaller f number, the more light it passes. You can use it in a more dimly lighted area and still get a good picture. When you are in a bright place you will use a larger f number.
If you tend to use the "green square" or the "P" mode mostly, of your camera, you truly can forget the rest!
@ebiggs1 wrote:" One would want to think that the 40mm is covered within the 18-55mm lens ..."
And it is.
The difference lies in the fact one is a zoom and the other is a prime. And than only because the f ratio is different. Again a number to tell you how much light will be transmitted to your camera. All you need to remember is, the smaller f number, the more light it passes. You can use it in a more dimly lighted area and still get a good picture. When you are in a bright place you will use a larger f number.
If you tend to use the "green square" or the "P" mode mostly, of your camera, you truly can forget the rest!
We don't usually dumb it down quite that much in this forum. The insinuation that Caroline won't understand the real explanations that are being attempted may be presumptuous.
And now she has both sides. She can either take Tim's excellent explanation or she can take mine. We strive to give people choices, don't we? 
