It may be a flap of fabric or an insulated tube. And its variations in construction are similar to those of the draft tube. The purpose of the collar is to prevent heat loss from around the neck. Warm air can be pushed out and cold air sucked in as the volume inside the bag changes. The collar acts as a dam to help slow this loss.
Collars are more important on colder weather bags.
They allow you to fine tune the warmth of the bag by redistributing insulation from top to bottom and vise versa. However, continuous baffles can allow down to shift on its own, so periodic shake-ups may be needed. Other bags are built with a side-block baffle. This is a netting wall running the length of the bag on the side opposite the zipper to hold the down in place. More fill is generally placed into the top of a bag since heat rises and since the insulation value in the bottom of the bag is reduced due to compression.