This has little to do with the type of lens. Just like your own eye, you're using the lens to focus the light flowing into your lens on a specific flat plane. the plane is either the film or your CCD sensor, but that doesn't make a difference.
Light comes from your source (sun or studio lights) and bounces off of the every object around you. Your lens collects that light and shines it (inverted) onto the picture plane. The problem is that big holes - or just an open square for the film produces an extremely blurry image. That's why pinhole cameras can create perfectly sharp images - when the light is forced through a small opening it's focused.
A modern glass lens is also focusing the image, but when you're using a lens, you're using two or more ground pieces of glass to focus the image. Depth of field is created, where only objects a fixed distance from the lens are in focus and others are out of focus.
So you're in a park, looking across at a young couple 10 feet away and behind them are some kids on a swing 30 feet away. You rotate the barrel of your lens to focus on the young couple. by turning the barrel of the lens, you've changed the distance between the pieces of glass in your lens, adjusting the focal point. Now the could starts walking towards you and they're no longer in focus.
If the couple moves towards or away from you the same effect is going to happen, they'll no longer be in focus because your lens isn't set for the distance where they are. If you have a high F-stop them your depth of field is reduced and there's more of the image that's in focus - allowing the couple to move more before they aren't in focus any more.
This is a basic physical principle of photography, it's great background but it's not really something you need to know to take good pictures. If this is for a class quiz, next time do your own research and learn things for your self. And I know the answer is long and involed, but deal. :P
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