"sling bag" could refer to many things, but in current colloquial american english the term refers to a bag designed much like a backpack but with a single strap that is worn over one shoulder and across the front of the torso instead of a strap for each shoulder, or a variation on such a design.
The main design goal is to provide faster, more convenient access to bag contents than a backpack with better load distribution than a shoulder bag. in many cases it is convenient to access contents while wearing the bag. to facilitate this some are designed to work "upside down". that is, when the bag is pulled around to the front of the body, the what had been the bottom when worn on the back, is now on top. side openings, front surface openings, novel closures, etc. facilitate this.
Such bags tend to be carried in a position similar to that of a backpack, which is higher than that of a messenger bag.