Box model only apply when container have a padding or a border?

Box model only apply when container have a padding or a border?

Take a look at this two links:
http://is.gd/i1In5L
http://is.gd/vAM8Kh
It is simple, when I apply border to the container, the child's padding is
considered by the container, if I don't apply border(or padding works as
well) it doesn't so when I apply background-color, there is a white space
between the document objects.
Can anybody explain this? is there a way to solve it without applying
padding or border to the container?