Actually, the S doesn't seem to stand for anything. Its just there. Don't pay it any mind; there have certainly been more confusing marketing schemes used for new products. What matters is how these products are different than their predecessors, and in this, there isn't much that needs to be understood. The S-series processors use less power than the previous Core 2 Quads. That's it. That's the only difference.
Which isn't to say that power savings is not important. In fact, saving power can be very important. Saving power keeps more money in your pocket, keeps wildlife happy, and reduces the amount of heat your computer has to exhaust. I think those are respectable traits for any piece of hardware, and Intel seems to agree, as they haven't simply released one product. Instead, they've duplicated three of their Core 2 Quad products buy unleashing the Q8200S , the Q9400S , and the Q9550S. All of these products have a 65W TDP, instead of 95W, which is standard for Core 2 Quads.