Spur gears are a type of cylindrical gear, with shafts that are parallel and coplanar, and tooth that are straight and oriented parallel to the shafts. They’re arguably the simplest and most common type of gear – 
simple to manufacture and suitable for a wide range of applications.
One’s teeth of a spur gear have got an involute profile and mesh one tooth at a time. The involute type implies that spur gears only generate radial forces (no axial forces), but the approach to tooth meshing causes high stress on the gear tooth and high noise production. Because of this, spur gears are typically used for lower rate planetary gearbox applications, although they can be used at nearly every speed.