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 positions itself 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; top: 10px; left: 20px;
Absolute position inside of a parent with relative position