position: absolute; left: 20px; top: 20px;
The only containing element for this div is the main window, so it positions itself in relation to it.
position: relative; top: 0; right: 0;
Relative position in relation to its siblings.
float: right; position: relative; top: 20px; left: 20px;
Relative to its sibling div above, but removed from flow of content.
position: absolute; bottom: 10px; right: 20px;
Absolute position inside of a parent with relative position
position: absolute; right: 0; left: 0; top: 200px;
Absolute position with both left and right declared