Hmmm... Good question. 
Can't tell you abt other's opinion, but I can tell you
what I personally think. 
To me, a shield is like a personal force-field that I erect around
me or an object with my energy or earth power or whichever energies
etc etc. It may be temporary or designed to last permanently to
deflect any negativity that may aim itself at me or attempt to
attach to me.
Warding serves the same purposes, only that for me, I'll see it
more as a magickal barrier that is anchored to a place or an object
etc using physical objects like runestones, bindrunes, crystals,
maybe even servitors. etc etc. The probabilities are quite
endless... Warding is, to me... designed to last for a certain
period of time. Although, with time, it can grow strong and
permanent and would actually require taking down when no longer
needed.
Thinking in terms of analogy. A knight going into battle. A shield
is the shield he carries, and a ward is the chainmail on him. If he
is tired or weak, he may drop the shield (shield will falter), but
unless it is removed (obj not recharged or physical anchor removed)
the chainmail will still be on him... 
Get the picture? P.S. Lousy analysis... I know... 
But can't think of anything
else right now... Heh. Srry.
