Pretty much everybody has recapped this correctly. Green tags mean that the bags will be available to the customer planeside, and orange tags mean the bags will be available to the customer at the baggage claim. It's the same ticket you get when you have to gate check a bag on mainline.

The poster who said that airports with jetways have bags delivered to the claim area is correct. In those cases, bags should be orange tagged. Why the difference, I really don't know. If you can't pick up a green-tagged bag planeside, where else can you get it? I suppose the idea is that if you're making a connection, you have to be able to through check your bag.

FYI, if you forget to have your bag green tagged coming into IAD and you're on a tight connection, you can ask the agents if they will orange tag it through to your destination, saving you a trip to the baggage claim.