DK (double knitting) or Light Worsted yarns are generally above fingering/sock weight, but below worsted or aran weight. They are often about the same size as "sport" weight yarns, but are not necessarily interchangeable. Be sure to check gauge carefully. The characteristics below come from the Craft Yarn Council's Guidelines.
DK yarns are great for mittens, hats, shawls, scarves, sweaters... anything that calls for a smaller-than-worsted weight yarn.
Guage: 21-24 stitches in stockinette over 4"
Recommended Needle Size: 3.75-4.5m (US 5-7)