For precise placement, use styles:

.img_abs {
position:absolute;
top=100; left=150;
...

Any element placed at an absolute position is treated as outside the HTML layout's control. It just doesn't exist. Text will run right over it, for example.

Relative positioning moves an element relative to the placement picked by the HTML layout mechanism. For example, you could add space between a table caption and the table below, this way:

<caption style="position:relative; top='-3'">Table Caption</caption>

That would move the caption up, relative to its default position above the table, three pixels.