I was shopping for yarn today at knitpicks, and I was just wondering what Dk weight was. I'm formiliar with all of the other weights.
It's between sport and worsted. Here's a good bookmark. http://www.yarnstandards.com/weight.html
Some of the yarn KPs has labeled as DK is actually a little heavier, like Merino Style. It's really a light worsted or worsted weight.