Why is camera warranty sometimes won't work on whole camera but on particles?

I'm thinking on buying an used Nikon D60 with two year warranty on it. Although it is used it still has the warranty on it, so if everything is wrong I think, I can still replaced it? I do not know alot about photography nor SLR cameras and about whole camera warranty!
Is camera warranty means that is a particle is broken I can replace it? I by the way is going to buy an used camera from a person who bought hers camera from henry's (Canadian camera store).Is buying an used camera with warranty is good idea or not?

Warranty affects anything in the camera. If a single component breaks down (for a reason that is not a user fault, such as dropping the camera or letting it get wet). Then the warranty will cover it.

Nikon has good warranty and customer service standards. If something in your camera won't work, well I think you have the right to replace it, or have it fix it.
Hope it helped!