Very overly simplistic explanation,
Advance, cam begins to open sooner and closes sooner also
Retard, cam begins to open later and closes later.
Overlap, this is the period when both valves are open slightly. Exhaust retard will increase overlap while advance will lower overlap.
In a low RPM situation, overlap can rob power by pulling good air/fuel out the exhaust.
In a high RPM situation, it can help make power by starting the intake pull sooner/faster.
So as Sailor touched on, many will retard the exhaust because you'll see more peak power from doing so (correctly); not necessarily a nicer curve though.