The knit sleeve band, also known as a knit cuff, is something seen on sweatshirts, jackets and coats.  The purpose is to make the bottom of the sleeve more fitted but still is very comfortable to wear.  Typically, sleeve bands are made with a ribbed knit, which just means the knit has defined lines or ribs.  You'll want these ribs to go parallel with the length of your arm.  This allows you to get the maximum stretch going around your wrist.  It's actually really easy to add knit bands to the bottom of your sleeves, even if you don't have a pattern.  This tutorial shows you what you need to know and gives you simple directions on creating your own knit sleeve bands.