The listing, 1 Dozen Hacky Sack Balls, Various Designs NIP* has ended.
The name Hacky Sack has become a genericized trademark, and can refer to either the footbag or a footbag game. The classic hacky sack circle consists of two or more players that keep the footbag off the ground without using their hands. Some common tricks include stalls, bag daggers, lifts, loops, ham spalts, and milk tosses.
A footbag is both a small, round bag, and the term for the various sports played with one -- characterized by controlling the bag by using one's feet. Although often referred to generically as a Hacky Sack, that is the trademarked name of one specific brand.
These would make great party favors, stocking stuffers, or just a great way to get those kids outside and off the computer and video games.
* The picture shows two full dozens, this listing is for one of those full large bags only.