First Example
If you place a series of boxes onto a screen then they will take up the default overlap. Usually the later ones will be set on top of the earlier ones.
If you want to change this default situation then you must use the fact that the position of an element is determined by its z-index, higher indices being on top of lower ones. Use of z-index to do this is illustrated in the second example.
To see what your computer screen would look when using the default positioning open this window
Here is the CSS code and the XHTML used to produce the layout above.
Notice the "shorthand" for CSS when a number of elements have code in common.