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