Positioning wasn't part of CSS-1, but is implemented by both MSIE 4 and Netscape 4. Relative positioning is inline and in-band. Absolute is relative to current context and out of band. Positioning establishes a new context for enclosed tags. Positioning allows, for example, overlapping:
<span style="font-size:72pt; font-family:sans-serif; color:green"> U<span style="position:relative; left:-.5em; top:.2em">O</span></span> is the UO logo
UO is the UO logo