Herringbone gear is a special types of gear which is a side to side (not face to face) combination of two helical gears of opposite hands. Unlike helical gears they can sustain axial load smoothly. From the top the helical grooves of this gear looks like letter V.
Herringbone gears can transmit high power and capable of taking axial load. They are used in heavy machineries. Helical gears are carefully made and of high precision. So the cost of helical gear is higher than the spur gears or helical gears.