Definition: On an RC vehicle, the spur gear is a large gear that meshes with the pinion gear on an electric motor or the clutch bell on an nitro engine. The pinion gear or the clutch bell turn the spur gear which in turn provides power to the drivetrain.
Spur gears come in different sizes -- number / spacing of teeth -- that matches up with certain sizes of pinion gears or clutch bells. The gear setup (specific sets of spur gears and pinion gears or clutch bells) affects the speed and power of the RC. Some gear setups provide more top end speed but may be slow off the line while others get up to speed faster but may not be the fastest over the long haul.
Tightening the spur gear is one adjustment that may help when attempting to do wheelies with your RC.
