DK tells you the weight of the yarn and the gauge you will get (how many stitches per inch) when you knit with it. The gauge helps you determine sizing of a garment and how much yardage you will need. It also affects the look of the garment -- a DK weight yarn is a lighter weight yarn than a worsted weight or a bulky yarn.
Yes, you can use DK yarn for "regular" knitting. But when you cast on 20 stitches in DK the width of your scarf will be narrower than if you cast on 20 stitches in a bulky yarn.