How does "selling wind power" work?

Today’s newspaper had an article saying that Ga. Power would be buying wind turbine power from a power company in Oklahoma. http://www.ajc.com/news/business/georgia-power-to-buy-wind-power-from-oklahoma/nXS8X/ I suppose that’s fine; they want to show how “green” they are.

But in real terms, what does this mean? When power is coursing across the wires, does it really have identity as “wind power” as opposed to coal power or nuclear power? How can I know that what is coming to my house is ‘clean’ power as opposed to something else?

My impression is that this is all a bookkeeping sleight of hand, and there’s now way to really know the source of power. And that the local utility is doing this for PR purposes rather than to actually display a commitment to renewable energy.

Does anyone have any facts about how this is done?