In possibly over hunted areas the DOW has limited the amount of licenses sold so that there are not 1000 people per square mile hunting all at once. The DOW has also limited hunting pressure in areas that can't support or don't support large hunt-able populations of a specific species.
All deer licenses are draw only (ie lottery). You can NOT got to the sporting goods store and buy an over the counter license. This is also the same for Big Horn Sheep, Mountain Goat, Bear and Moose.
There are usually left over licenses available after the draw but need to be purchased at the DOW offices. Left over licenses are usually available for Deer, Elk and Bear. Unfortunately they will probably not be for the area you wish to hunt... at least that has been my experience.
Does this clear things up or make it mud?