Round and Square refer to the type of drill (aka diamond) that is used to create the mosaic design.
Round drills are circular and typically result in a more shiny finish, are quicker to complete but are sometimes less detailed designs as the drills don\'t fill the entire design due to their round nature.
Square drills are square and will fix together side by side. Because they are placed flush against each other, they can create a more complete design. However, square designs are slower to complete as you need to be more aware of placement to ensure the drills are aligned straight.