Snowballs! This is a bit of a toy to cause antics that blizzard added into the game. Originally you could only make snowballs if you were an engineer and had the proper schematic and material. You can find more details about that on the showball Thottbot page. After a patch they added them to the PVP area Alterac Valley. Snowballs thrown in here pack an extra punch which you can use to your advantage in PVP as shown below in the videos. Keep in mind these special snowballs will disappear after you've left Alterac Valley and you can only hold one stack of five while in this PVP area.